A critical mechanism underpinning current biological diversity is the extent to which one species mates with, or avoids mating with, another. However, little is known about the factors that mediate hybridisation, especially during the initial and rarely observed stages of secondary contact when interspecific interactions have not responded to selection. In particular, whilst hybridisation is ultimately a behavioural phenomenon, the role of behaviour in mediating hybridisation and how it is influenced by environmental and circumstantial factors is rarely investigated. Recently introduced species provide us with unequalled opportunities to study these factors. In this thesis I examine the role of behavioural mechanisms, in particular male-male competition and mate choice, in mediating mating patterns between two genetically and phenotypically distinct lineages of European wall lizard (Podarcis muralis) that have come into recent secondary contact through human introductions. In Chapter Two, I investigated how sexual selection during allopatry is responsible for creating stark differences in phenotypic traits such as body size and weapon performance evident in the two lineages today, ultimately explaining the strong biases in dominance during territorial disputes between males. However, I also show that even given this asymmetry in male competitive ability, the extent to which it extrapolates into greater access to females in naturalistic, outdoor enclosures depends strongly on the spatial clustering of basking sites, a critically important resource for many ectotherms. In contrast to initial predictions suggested by asymmetries in male competition outlined in the previous chapter, in Chapter Three I show that both paternity and courtship behaviour was strongly assortative in the outdoor enclosures. Further investigation through staged experiments on olfactory mate choice, mating trials and analyses on specific behavioural data obtained in an enclosure experiment, I show that lineage based dominance actually contributes to assortative mating patterns in conjunction with weak conspecific male choice. In contrast, female choice seems to play no role in mediating the mating patterns observed between the two lineages. In Chapter Four I had the rare opportunity to examine the morphological and behavioural factors that predict why animals should hybridise in the first place, using the data obtained in the enclosure experiment above. I found that hybridisation was particularly common between small individuals of the larger lineage and large individuals of the smaller lineage; a result that corroborates the mechanisms determining the assortative patterns uncovered in Chapter Three. Additionally, hybridisation rates were particularly high in less dominant individuals, which I suggest is due to subordinate males having reduced opportunities for courting conspecific females due to male-male competition, requiring them to become less ‘choosy’ and therefore more likely to mate with heterospecifics. Finally, secondary contact cannot occur without at least one lineage coming into a new environment, and yet relatively little attention is paid to how this environmental change can affect the signals involved in intraspecific communication and mate choice. In Chapter Five I show that a change in the amount of time male lizards spend thermoregulating (a likely consequence of arriving in a new environment) significantly changes the chemical composition of their scent marks. However, whilst female lizards were able to detect these effects, they did not seem to base their mating decisions on them. Nevertheless, this result raises interesting questions about the potential function and consequences of this plasticity, and highlights the importance of considering plasticity in chemical communication in heterogeneous environments. Overall, this thesis shows the critically important role of behaviour in mediating intra- and interspecific mating patterns during recent secondary contact. In particular, it highlights how the direction and extent of hybridisation and competition are influenced by the degree to which differing morphological and behavioural phenotypes interact over a heterogeneous environment, particularly during the initial stage of secondary contact when mate choice has not had the chance to respond to the selective pressures of hybridisation.

Among the different sensory modalities that play a role in sexual selection, acoustic communication plays an important one. Acoustic communication has been known to be used for male-male competition (territory maintenance, male aggression during mating),for advertisement to the opposite sex (mating status, body condition, genetic quality, nutritional status) and used by females to sample and choose conspecific preferred males. The use of acoustic communication for sexual display and information exchange has been extensively studied in multiple taxa, including insects, anurans, birds and mammals. Among insects, crickets have proven to be good model systems to study sexual selection based on acoustic communication as most species have an elaborate acoustic communication system, male advertisements, diverse types of mating incentives for females (such as glandular feeding) and a female dominated mating system. Generally, in crickets males produce species-specific calls which are used by females to localize conspecific males. Besides, calls show high levels of intraspecific variation and are energetically costly to produce. Moreover, as in crickets predominantly the females show phonotaxis towards male calls, calls also can play a role in mate sampling and choice by acting as indicators of preferred male quality. Despite being studied for many decades there are certain gaps in the studies examining mate choice in crickets. Some of them are, lack of understanding of the variation of male calling traits in nature and its role in signal evolution, lack of understanding regarding the ecological context of mate sampling and the evolution of alternative mating strategies. Hence, the tree cricket Oecanthus henryi was chosen as a study system to address these gaps in the understanding of female choice based on acoustic signals. In the tree cricket Oecanthus henryi, males call and females use calls to localize conspecific males and hence potentially females can choose males based on acoustic cues. To understand the evolution of female preference for male acoustic cues it is important to understand the variation in the calling songs in the field and identify repeatable call features that are reliable indicators of preferred male traits (morphological, developmental or genetic). I measured repeatability of male call traits in the field to understand their variation, reliability and consistency. Carrier frequency was the only call trait that was highly repeatable and hence was reliable and consistent. Following this I examined whether any of these call traits were indicators of male morphological traits (such as male size and fluctuating asymmetry) which are known to be preferred by females. It was found that carrier frequency was negatively correlated with body size; hence carrier frequency was both reliable and indicated male size. I also found that females preferred larger males during mating, as revealed by the longer mating durations and longer spermatophore retention time. Interestingly, though this study indicated that females could in principle use lower call carrier frequency to localize preferred larger males, simultaneous choice experiments done in the laboratory revealed that the females do not use this cue. These contrasting results may be because females are incapable of discriminating small differences in frequency or because they use non-acoustic cues for mate choice. However, whichever cues the females use to discriminate between males in the laboratory conditions, often these preferences are not realized in the field. The main reason behind this is that searching for preferred mates in the field can be costly and this might force females to choose sub-optimal males. Theoretical models predict that male movement and spacing in the field should influence female sampling tactics and in turn, females should drive the evolution of male movement and spacing to sample them optimally. Moreover, simultaneous sampling of males using the best-of-n or comparative Bayes strategy should yield maximum mating benefits to females. Many of the theoretical mate sampling strategies involves recall of the quality and location of individual males, which in turn requires male positions to be stable within a night. Calling males of O. henryi showed high site fidelity within a night, potentially enabling female sampling strategies that require recall. To examine the possibility of simultaneous acoustic sampling of males, I estimated male acoustic active spaces using information on male spacing, call transmission and female phonotactic threshold. Males were found to be spaced far apart and active space overlap was rare. I then examined female sampling scenarios by studying female spacing relative to male acoustic active spaces. Only 15% of sampled females could hear multiple males, suggesting that simultaneous mate sampling is rare in the field. Moreover, the relatively large distances between calling males suggest high search costs, which may favor threshold strategies that do not require memory. Using the insights gathered from these two studies I examined a unique calling behaviour from leaf holes, baffling, observed in this species. Baffling behaviour has been found in multiple species of the genus Oecanthus where the males call from selfmade holes in leaves rather than calling from leaf edges (their natural calling surface) thus increasing their loudness many fold. I started by examining the natural history of baffling and found that baffling is an extremely rare behaviour in the field. However field observations and laboratory experiments revealed that many males can baffle and hence it is not an obligatory behaviour shown only by a few males. It was hypothesized that one reason for the rarity of baffling could be resource limitation. It was found that baffling males prefer larger leaves possibly due to higher SPL gains achieved by baffling on the larger leaves, which is a limited resource in the field. However this alone was insufficient to explain extreme rarity of bafflers in the field. Hence I examined which males were using this behaviour in the field. Using field observations and laboratory experiments it was found that less preferred males (smaller and quieter) baffled more which provided them with higher calling SPL and greater sound-field volume and thus a higher number of potential mates. Moreover, baffling also increased the mating duration for the less preferred males thus providing more time to these males for sperm transfer. The females could not differentiate between an inherently loud caller and a caller whose SPL was increased artificially (as if it was baffling). Hence I concluded that baffling is probably a cheater strategy used by the less preferred males to fool the females into approaching them and mating for longer durations. To my knowledge, this is the first study that has estimated male call variation in the field to understand its role in female choice in tree crickets. Moreover this is also the first study to examine the ecological context of mate choice in tree crickets. This is also the first study to examine the advantages of baffling behaviour and its potential evolutionary implications. the different sensory modalities that play a role in sexual selection, acoustic communication plays an important one. Acoustic communication has been known to be used for male-male competition (territory maintenance, male aggression during mating),for advertisement to the opposite sex (mating status, body condition, genetic quality, nutritional status) and used by females to sample and choose conspecific preferred males. The use of acoustic communication for sexual display and information exchange has been extensively studied in multiple taxa, including insects, anurans, birds and mammals. Among insects, crickets have proven to be good model systems to study sexual selection based on acoustic communication as most species have an elaborate acoustic communication system, male advertisements, diverse types of mating incentives for females (such as glandular feeding) and a female dominated mating system. Generally, in crickets males produce species-specific calls which are used by females to localize conspecific males. Besides, calls show high levels of intraspecific variation and are energetically costly to produce. Moreover, as in crickets predominantly the females show phonotaxis towards male calls, calls also can play a role in mate sampling and choice by acting as indicators of preferred male quality. Despite being studied for many decades there are certain gaps in the studies examining mate choice in crickets. Some of them are, lack of understanding of the variation of male calling traits in nature and its role in signal evolution, lack of understanding regarding the ecological context of mate sampling and the evolution of alternative mating strategies. Hence, the tree cricket Oecanthus henryi was chosen as a study system to address these gaps in the understanding of female choice based on acoustic signals. In the tree cricket Oecanthus henryi, males call and females use calls to localize conspecific males and hence potentially females can choose males based on acoustic cues. To understand the evolution of female preference for male acoustic cues it is important to understand the variation in the calling songs in the field and identify repeatable call features that are reliable indicators of preferred male traits (morphological, developmental or genetic). I measured repeatability of male call traits in the field to understand their variation, reliability and consistency. Carrier frequency was the only call trait that was highly repeatable and hence was reliable and consistent. Following this I examined whether any of these call traits were indicators of male morphological traits (such as male size and fluctuating asymmetry) which are known to be preferred by females. It was found that carrier frequency was negatively correlated with body size; hence carrier frequency was both reliable and indicated male size. I also found that females preferred larger males during mating, as revealed by the longer mating durations and longer spermatophore retention time. Interestingly, though this study indicated that females could in principle use lower call carrier frequency to localize preferred larger males, simultaneous choice experiments done in the laboratory revealed that the females do not use this cue. These contrasting results may be because females are incapable of discriminating small differences in frequency or because they use non-acoustic cues for mate choice. However, whichever cues the females use to discriminate between males in the laboratory conditions, often these preferences are not realized in the field. The main reason behind this is that searching for preferred mates in the field can be costly and this might force females to choose sub-optimal males. Theoretical models predict that male movement and spacing in the field should influence female sampling tactics and in turn, females should drive the evolution of male movement and spacing to sample them optimally. Moreover, simultaneous sampling of males using the best-of-n or comparative Bayes strategy should yield maximum mating benefits to females. Many of the theoretical mate sampling strategies involves recall of the quality and location of individual males, which in turn requires male positions to be stable within a night. Calling males of O. henryi showed high site fidelity within a night, potentially enabling female sampling strategies that require recall. To examine the possibility of simultaneous acoustic sampling of males, I estimated male acoustic active spaces using information on male spacing, call transmission and female phonotactic threshold. Males were found to be spaced far apart and active space overlap was rare. I then examined female sampling scenarios by studying female spacing relative to male acoustic active spaces. Only 15% of sampled females could hear multiple males, suggesting that simultaneous mate sampling is rare in the field. Moreover, the relatively large distances between calling males suggest high search costs, which may favor threshold strategies that do not require memory. Using the insights gathered from these two studies I examined a unique calling behaviour from leaf holes, baffling, observed in this species. Baffling behaviour has been found in multiple species of the genus Oecanthus where the males call from self- made holes in leaves rather than calling from leaf edges (their natural calling surface) thus increasing their loudness many fold. I started by examining the natural history of baffling and found that baffling is an extremely rare behaviour in the field. However field observations and laboratory experiments revealed that many males can baffle and hence it is not an obligatory behaviour shown only by a few males. It was hypothesized that one reason for the rarity of baffling could be resource limitation. It was found that baffling males prefer larger leaves possibly due to higher SPL gains achieved by baffling on the larger leaves, which is a limited resource in the field. However this alone was insufficient to explain extreme rarity of bafflers in the field. Hence I examined which males were using this behaviour in the field. Using field observations and laboratory experiments it was found that less preferred males (smaller and quieter) baffled more which provided them with higher calling SPL and greater sound-field volume and thus a higher number of potential mates. Moreover, baffling also increased the mating duration for the less preferred males thus providing more time to these males for sperm transfer. The females could not differentiate between an inherently loud caller and a caller whose SPL was increased artificially (as if it was baffling). Hence I concluded that baffling is probably a cheater strategy used by the less preferred males to fool the females into approaching them and mating for longer durations. To my knowledge, this is the first study that has estimated male call variation in the field to understand its role in female choice in tree crickets. Moreover this is also the first study to examine the ecological context of mate choice in tree crickets. This is also the first study to examine the advantages of baffling behaviour and its potential evolutionary implications.

African mole-rats (Bathyergidae) exhibit a wide range of social structures ranging from solitary to eusocial. This allows for studies looking at links between sociality and measurable characteristics such as spatial learning and kin-recognition. Furthermore, the existence of species with differing level of sociality allows for comparison between the highly social species and the solitary species. The existence of differences in spatial learning ability and memory between the sexes has long been debated. Eusocial Damaraland mole-rats (Cryptomys damarensis) and solitary Cape mole-rats (Georychus capensis) were tested to see if there were sex or species differences in the ability to locate food in an artificial maze task with the express purpose of investigating spatial learning and memory. Measurements of the time taken to complete the task, the distance travelled, wrong turns taken, and the average velocity at which animals travelled were used to compare performance between animals. Both sexes in each of the species exhibited learning and a decay in memory over time. The Damaraland mole-rat exhibited superior learning and memory retention when compared to the Cape mole-rat. Male Cape mole-rats had superior learning and longer term memory retention when compared to females of the same species. There was no significant difference in learning curves between male and female Damaraland mole-rats, but this species did exhibit a tendency for females to show better medium term memory retention while males performed better on long term memory trials. Species differences are likely to be linked to social organization and possibly the resultant burrow-structure in the natural environment, while sex-differences may be due to differing life histories. Kin-recognition is important in maintaining the social structure and hierarchy in the eusocial species of African mole-rat, Cryptomys damarensis. Opposite sex sibling pairs from reproductively quiescent colonies were tested to see if exposure to colony urine odour would reinforce recognition of opposite sex siblings and the concomitant incest avoidance. Control sibling pairs from the same colonies were exposed to water. Mating, social, and non-social behaviours were measured in all sibling pairs. In addition, urinary levels of cortisol, progesterone and testosterone were measured to examine the interaction between four factors: colony olfactory cues (urinary odour), hormone levels, mating behaviour and relatedness. Exposure to urinary odour reinforced recognition and was correlated to a decrease in mating behaviour. Hormonal assays suggest that female hormone levels are modified based on exposure to colony urine odour, while males are not affected. Olfactory cues such as colony urinary odour is linked to the alteration and correlation of hormone levels and mating behaviour. The Damaraland mole-rat and other species in the family Bathyergidae provide a useful system for investigating aspects of learning and memory, as well as the potential correlation between cognitive processes and sociality. / Dissertation (MSc (Zoology))--University of Pretoria, 2007. / Zoology and Entomology / unrestricted

Orientador: Célio Fernando Baptista Haddad / Resumo: As forças de seleção natural e sexual atuam sobre os indivíduos e podem os levar ao isolamento reprodutivo. O acúmulo de modificações evolutivas pode levar ao surgimento de novas entidades. Espécie é unidade basal em biologia; assim, com sua exuberante biodiversidade, a Mata Atlântica é uma floresta neotropical fundamental para estudos de processos evolutivos em anfíbios anuros. Reunindo os gêneros Cycloramphus, Thoropa e Zachaenus, a família Cycloramphidae agrupa espécies de anuros saxícolas e terrestres, todas endêmicas da Mata Atlântica brasileira. Pelos seus hábitos especializados de vida e reprodução, os cicloramfídeos formam um grupo atrativo para estudos evolutivos. Na presente tese são apresentados quatro capítulos que abordam três escalas distintas da evolução: a diversificação entre espécies, populações e indivíduos. São abordados: (1) a filogenia molecular multilocus de Cycloramphus-Zachaenus, demonstrando que a evolução da terrestrialidade no clado se correlaciona com o dimorfismo sexual em tamanho; (2) a diversificação fenotípica associada a diversificações genéticas em populações de C. boraceiensis e C. dubius, demonstrando que estas duas espécies-irmãs saxícolas apresentam zona de contato e hibridação; (3) o sistema de acasalamento promíscuo e as relações sociais em C. boraceiensis, demonstrando que tamanho das fêmeas e dos machos e parentesco genético influenciam a formação dos casais; e (4) o sistema de acasalamento poligínico com fidelidade na espécie saxíco... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Natural and sexual selection forces act on individuals and may lead to reproductive isolation. The increase in evolutionary modifications may lead to the emergence of new entities. Species is the basal unity in biology; thus, with its exuberant biodiversity, the Atlantic rainforest is a fundamental Neotropical forest for studies of evolutionary processes in anuran amphibians. Comprising the genera Cycloramphus, Thoropa, and Zachaenus, the family Cycloramphidae gathers saxicolous and terrestrial anuran species, all endemic to the Brazilian Atlantic rainforest. Because of their specialized life and reproductive habits, cycloramphids are an attractive group for evolutionary studies. In the present thesis we show four chapters that address three distinct scales of evolution: the diversification among species, populations, and individuals. We goaled: (1) the multilocus molecular phylogeny of Cycloramphus-Zachaenus, demonstrating that the evolution of terrestriality in the clade is correlated with the sexual size dimorphism; (2) the phenotypic diversification associated with the genetic diversifications in populations of C. boraceiensis and C. dubius, demonstrating that these two saxicolous sister species show a contact zone and hybridization; (3) the promiscuous mating system and the social relations in C. boraceiensis, demonstrating that female and male sizes, and mate relatedness influence pair formation; and (4) the polygynic mating system with fidelity in the saxicolous specie... (Complete abstract click electronic access below) / Doutor

The aim of the project was to test female advantages of promiscuous and/or polyandrous mating. Although, previous studies reported improved reproductive success of females copulating with multiple males in many model species, including Eublepharis macularius, this study failed to demonstrate such a benefit of polyandry. Traits reflecting female reproductive success, i.e. the number of produced eggs, egg mass, hatchability, hatchling mass, growth and survival were not influenced by copulation with multiple partners, but some of them were dependent on female condition. Only initial stages of juvenile growth were influenced by female identity and/or investment.

Natural populations exhibit a non-random mating behavior and it is assumed that mate preferences causing the non-random mating behavior play a role in sympatric speciation. In my thesis, I have proposed a new model of a mate preference that is based on ecological performance and I have named it fitness-based mating. Individuals that express this mate preference choose primarily fit partners. Fitness-based mating is modelled for haploid, diallelic populations. Individuals are distributed across two niches, and genomes are simplified to two loci. The first locus is subject to natural selection, and the second-locus genotype gives the strength of the mate preference. The population is separated into females and males, among which only females exhibit the mate preference. Ecological selection acts on both sexes alike. With the model I have investigated how female choosiness based on direct advantages offered by their partners can cause and maintain a polymorphic population. Fitness-based mating is an evolutionary successful mating strategy. It spreads in a population due to its amplifying effect on the reproductive success and on the attractiveness of its carriers. A polymorphism arises naturally in the model. The emergence of a stable polymorphism of traits underlying ecological selection is of special interest, because a polymorphism can be a precursor of speciation.

Educational assortative mating and earnings inequality has both increased in both Europe and the United States in the last decades. As a result, educational assortative mating, or educational homogamy, has been suggested as a potential explanation for the increase in earnings inequality. According to this hypothesis increased sorting on education will lead to polarization between lower and higher-educated couples where the advantages of the latter will compound on one another and lead to increased economic inequality.   The majority of the studies to date report a non-relationship between educational assortative mating and earnings inequality, one of the exceptions being a study of Denmark. This exception has led sociologists to theorize that the impact of educational assortative mating could be especially strong in the Nordic countries. In this study I test this hypothesis by employing a novel decomposition method, the Theil-index, to answer if increases in educational assortative mating are associated with increases in earnings inequality in Sweden between 2000 and 2010, using data from the Standard of Living Survey (LNU).   The result is a non-relationship between homogamy and earnings inequality and an overall decrease in earnings inequality in the sample. The result is another null result for the hypothesis that educational homogamy leads to inequality, and points to a larger discrepancy between singles and couples than between couples. If corroborated, this decrease in earnings inequality would mean a divergence, in earnings inequality, between partnered individuals and the general population. Future studies should focus on the extent of this divergence.

Insular fauna face disproportionate risks of extinction owing to direct human perturbation and intrinsic factors that are enhanced at small population sizes. Currently, our understanding of the processes that promote long-term persistence of naturally small populations and the cryptic processes that may contribute to accelerating their decline is limited by lack of empirical investigations across the range of natural conditions. Implementing effective protections for rare and understudied taxa requires the identification and examination of factors that limit recruitment at critical life stages. Predicting population health outcomes of future perturbations further necessitates an understanding a taxon’s behavioral ecology. Finally, cryptic threats to viability, such as inbreeding depression, must be investigated with an appreciation for taxon-specific life history, as these attributes can alter the context in which severe fitness reductions are expressed. In this project I enlist integrative and cross-disciplinary approaches to study the behavioral ecology and conservation genetics of a critically endangered West Indian Rock Iguana, Cyclura nubila caymanensis, on Little Cayman Island. I demonstrate how coastal communal nesting areas, a critical limiting resource on the island, serve a diverse population demographic and contribute to significantly enhanced nesting outcomes. These data emphasize the importance of expanding protections for major sites, as aggregative nesting appears to be perpetuated by both habitat suitability and adaptive fitness benefits. I next evaluate the possibility of evolved inbreeding avoidance strategies, including natal dispersal, non-assortative mate choice, and genetic bet-hedging. I conclude that the contribution of pre-reproductive dispersal to inbreeding avoidance likely outweighs that of active mate choice. Importantly, observed patterns of siring success imply constrained female choice and sexual conflict over genetic mating outcomes – a pattern that may extend to many territorial, male-driven mating systems and therefore should be an important consideration in genetic management. Finally, I investigate age-dependent inbreeding effects and the degree to which inbreeding depression may limit recruitment to the breeding population. I fail to reveal significant correlations of multi-locus heterozygosity with hatchling fitness; however, negative effects of parental inbreeding on fecundity and hatching success imply fitness consequences of inbreeding depression could be felt at other life stages.

Assortative Mating, the tendency for mate selection to occur on the basis of similar traits, plays an essential role in understanding the genetic variation on academic achievements and intelligence (IQ). It is an important mechanism explaining spousal concordance. We used principal component analysis (PCA) for spousal correlation. There is a significant positive correlation between spouses by the new variable PC1 (correlation coefficient=0.515, p<0.0001). We further research the genetic factor that affects IQ by using the same data. We performed a low density genome-wide association (GWA) analysis with a family-based association test to identify genetic variants that associated with intelligence as measured by WAIS full-score IQ (FSIQ). NTM at 11q25 (rs411280, p=0.000764) and NR3C2 at 4q31.23 (rs3846329, p=0.000675) were 2 novel genes that haven't been associated with IQ from other studies. This study may serve as a resource for replication in other populations and a foundation for future investigations.

Stagonospora nodorum blotch (SNB), is a major foliar disease of wheat in the mid-Atlantic U.S., is caused by the necrotrophic fungus Parastagonospora nodorum. SNB is managed using cultural practices, resistant varieties, and foliar fungicides. There are increasing trends of severity and incidence of SNB in Virginia and the surrounding mid-Atlantic region, but it is not known if changes in the pathogen population are contributing to this trend. The overall goal of this research was to 1) determine the occurrence of quinone outside inhibitor (QoI) resistance in Virginia populations of P. nodorum infecting wheat, 2) quantify the distribution of G143A mutations conferring fungicide resistance in Virginia populations of P. nodorum, and 3) characterize genetic diversity of P. nodorum populations in Virginia and assess influences of cultivars and environments on population structure and SNB severity. For Objective 1, QoI resistant isolates of P. nodorum were identified from Virginia wheat fields, and this was the first report of QoI resistant P. nodorum in the United States. The G143A substitution in the cytochrome b gene of P. nodorum was associated with reduced QoI sensitivity, and in Objective 2, a state-wide, two-year survey of P. nodorum populations in Virginia determined that the G143A mutation was widespread in the state and among sampled fields the frequency ranged from 5-32% (mean = 19%). For Objective 3, P. nodorum was isolated from five different wheat cultivars across seven locations over two years in Virginia. SNB severity varied by cultivar but greater differences in disease severity were observed among locations and years suggesting environment plays an important role in SNB development. Among the necrotrophic effector (NE) genes examined, SnTox1 was predominant followed by SnTox3, and frequencies of NE genes did not vary by cultivar or location. P. nodorum populations in Virginia had high genetic diversity, but there was no genetic subdivision among locations or wheat cultivars from which individuals were isolated. Results also indicated that the P. nodorum population in Virginia undergoes a mixed mode of reproduction, but sexual reproduction made the greatest contribution to population structure. Overall, this work provides insights into the population biology of P. nodorum in Virginia and information on variability in fungicide sensitivity and cultivar susceptibility to SNB that has implications for the current and future efficacy of fungicides and host resistance for management of SNB. / Doctor of Philosophy / Wheat (Triticum aestivum L.) is one of the major cereal crops grown worldwide for food, feed, and other products. However, yields of this crop are often limited by fungal diseases including Stagonospora nodorum blotch (SNB) caused by Parastagonospora nodorum. Increasing trends of severity and incidence of SNB may be due to reduced sensitivity of P. nodorum to fungicides or increased virulence of P. nodorum populations on commonly grown cultivars. Fungicides such as quinone outside inhibitors (QoIs) are one of the major classes of fungicides used for disease control and G143A substitution is the most common point mutation associated with complete resistance to QoIs. Therefore, the overall goal of this research was to better understand genotypic and phenotypic variation in Virginia populations of P. nodorum in the context of fungicide sensitivity and susceptibility of wheat cultivars to SNB. The specific objectives were to 1) determine the occurrence of quinone outside inhibitor (QoI) fungicide resistance in Virginia populations of P. nodorum infecting wheat, 2) quantify the distribution of G143A mutations conferring QoI fungicide resistance in Virginia populations of P. nodorum, and 3) characterize genetic diversity of P. nodorum populations in Virginia and assess influences of cultivars and environments on population structure and SNB severity. Results from this research indicate that QoI fungicide resistance occurs in Virginia populations of P. nodorum due to a target site mutation (G143A substitution in the cytochrome b gene), and this mutation is widespread and relatively common in Virginia wheat fields. Based on a multi-year multilocation study, P. nodorum populations were genetically diverse, but there was no genetic subdivision among locations or wheat cultivars. SNB severity varied by location and cultivar, but disease severity was greatest at site-years with moderate springtime temperatures and high rainfall. Overall, this work contributes to a better understanding of P. nodorum populations including the current efficacy of fungicides and host resistance for management of SNB in the region.