No description available.

Philosophiae Doctor - PhD / Cichlids are one of the most diverse groups of fishes in the world. The highest concentration of cichlids is found in Lake Malawi, where over 1500 species are recorded to occur, which are dominated by the haplochromine tribe. Several authors proposed that sexual selection has been a major contributor in the high rate of speciation of haplochromine cichlids. However, sexual selection alone may not be sufficient to fully explain high speciation rates among cichlids. Many closely related cichlids in Lake Malawi differ only in coloration, yet occur in sympatry. It is still not fully understood how they remain reproductively isolated. Previous studies suggest that visual cues are primarily used for reproductive isolation and species recognition. In the current study, visual, chemical and acoustic cues were investigated to observe how they may potentially influence species recognition and reproductive isolation between two closely related species, Metriaclima estherae and M. callainos, as well as between alternate colour morphs of M. estherae.

Despite over seven decades of speciation research and 25 years of phylogeographic studies, a comprehensive understanding of mechanisms that generate biological species remains elusive. In temperate zones, the pervasiveness of range fragmentation and subsequent range expansions suggests that secondary contact between diverging lineages may be important in the evolution of species. Thus, such contact zones provide compelling opportunities to investigate evolutionary processes, particularly the roles of geographical isolation in initiating, and indirect selection against hybrids in completing (reinforcement), the evolution of reproductive isolation and speciation. The spring peeper (Pseudacris crucifer) has six well-supported mitochondrial lineages many of which are now in secondary contact. Here I investigate the evolutionary consequences of secondary contact of two such lineages (Eastern and Interior) in Southwestern Ontario using genetic, morphological, acoustical, experimental, and behavioural evidence to show accentuated divergence of the mate recognition system in sympatry. Mitochondrial and microsatellite data distinguish these two lineages but also show ongoing hybridization. Bayesian assignment tests and cline analysis imply asymmetrical introgression of Eastern lineage nuclear markers into Interior populations. Male calls are divergent between Eastern and Interior allopatric populations and show asymmetrical reproductive character displacement in sympatry. Female preference of pure lineage individuals is also exaggerated in sympatry, with hybrids showing intermediate traits and preference. I suggest that these patterns are most consistent with secondary reinforcement. I assessed levels of post-zygotic isolation between the Eastern and Interior lineages using a laboratory hybridization experiment. Hybrid tadpoles showed equal to or greater fitness than their pure lineage counterparts, but this may be countered through competition. More deformities and developmental anomalies in hybrid tadpoles further suggest post-zygotic isolation. Despite evidence for pre-mating isolation between the two lineages, isolation appears incomplete (i.e. hybridization is ongoing). I hypothesize that potentially less attractive hybrids may circumvent female choice by adopting satellite behaviour. Although mating tactics are related to body size, genetic status may play a role. I show that pure Eastern males almost always engage in calling, while hybrids adopt a satellite tactic. An absence of assortative mating, despite evidence of female preference, suggests successful satellite interception possibly facilitating introgression. / Thesis (Ph.D, Biology) -- Queen's University, 2013-03-04 16:01:33.892

Barriers to interbreeding limit gene flow between sister taxa, leading to reproductive isolation and the maintenance of distinct species. These barriers come in many forms, and can act at different stages in the reproductive process. Pre-copulatory barriers may be due to individuals discriminating against heterospecifics in mate choice decisions. These decisions may be informed through a range of sensory modalities. If a female is mated and inseminated, then there may be multiple postmating-prezygotic barriers that affect the success of heterospecific sperm in attaining fertilisations. Post-zygotic barriers can be very early acting, resulting in embryonic fatality, or may be later acting, affecting the fitness of hybrid offspring. In this thesis I investigate potential reproductive barriers between the interbreeding field crickets Gryllus bimaculatus and G. campestris. I find that females of both species show only weak preference for conspecific calling song, and may even respond phonotactically to songs typical of heterospecific males. Female G. bimaculatus are repeatable in their preferences and strength of response. G. bimaculatus females presented with synthetic songs prefer those with longer inter-pulse intervals, whereas G. campestris show no discrimination between these songs. Upon meeting, G. campestris females strongly discriminate against heterospecific males, behaving aggressively towards them. This is likely driven by females responding to close range species recognition cues, including chemoreception. The species differ in their cuticular hydrocarbon profiles, and females that are no longer able to use their antennae to receive chemosensory information reduced their aggressive behaviour towards heterospecific males. G. bimaculatus females will mate with heterospecific males, though less readily than to conspecifics. When sequentially mated to both conspecific and heterospecific males, these females will preferentially take up and store sperm from the conspecific male, and sperm from conspecific males is more likely to sire offspring than would be predicted from the proportion of sperm in storage. Eggs from inter-species mating pairs are less likely to begin embryogenesis, and are more likely to suffer developmental arrest during the early stages of embryogenesis. However hybrid embryos that survive to later stages of development have hatching success similar to that of pure-bred embryos. After mating, phonotaxis of G. bimaculatus females towards male songs follows a pattern of suppression and subsequent recovery, likely triggered through detection of seminal proteins transferred in the male ejaculate, or detection of mechanical filling of the spermatheca. This pattern of suppression and recovery of phonotaxis does not differ between females mated to conspecific or heterospecific males. Females that lay few or no eggs do not experience a refractory period.

Examining the ecological processes generating evolutionary patterns is crucial to understanding how biodiversity arises and evolves. One of the most striking examples of evolutionary diversification is provided by the flowering plants (angiosperms) and their flowers. Pollinators are traditionally considered to be the most important selective agents and drivers of floral diversity. However, many angiosperms have a generalized floral morphology and are visited by a diverse and overlapping suite of pollinators, making it unclear how pollinators could have driven diversification in these taxa. In addition, flowers and plant reproductive success are likely to be influenced by factors other than pollinators, such as herbivores, precipitation, and temperature. These factors need to be considered along with pollinators in order to improve our understanding of angiosperm evolution and diversification. In my thesis, I focussed on the processes influencing adaptation and diversification in flowering plants in the genus Mertensia (Boraginaceae), which have relatively unspecialized flowers that attract a variety of nectar- and pollen-feeding insects. In Chapter One, I explored correlations among floral traits, vegetative traits, and flowering phenology across 12 Mertensia species. In Chapter Two, I assessed reproductive isolating barriers between related Mertensia species occurring in sympatry. In Chapter Three, I examined the ecological function of floral orientation in two Mertensia species with respect to pollinators and precipitation. First, across Mertensia species, I found that early-flowering species were shorter, produced fewer flowers, and occurred at higher altitudes than late-flowering species—suggesting a life-history trade-off between plant size and flowering phenology, as well as an altitudinal effect on both traits. Interspecific variation in floral traits was not strongly associated with variation in flowering phenology or plant size. Second, between sympatric M. brevistyla and M. fusiformis populations, I found weak reproductive isolating barriers and possible hybridization. Most pre-pollination barriers were weak, as the two Mertensia species shared similar habitats, flowering phenology, and pollinator assemblages. The two relatively strong barriers were floral (ethological and mechanical) isolation and post-pollination isolation: Pollinators transferred significantly more of a pollen analogue among conspecific than heterospecific plants in mixed-species arrays, and flowers yielded higher seed set when receiving conspecific rather than heterospecific pollen in hand-pollination experiments. Lastly, I found that floral orientation was more likely to be under selection by precipitation than by pollinators, in that paternal fitness (i.e., pollen germination) was reduced by contact with water and that pollinator-mediated selection via maternal fitness (i.e., seed set) was not detected. A more pendant floral orientation likely protects the relatively long and exposed anthers of M. fusiformis from rain, while the less pendant M. brevistyla does not require this protection because of its shorter, more concealed reproductive structures. Although I detected an effect of floral orientation on seed set, I was not able to identify the selective agents driving this effect. In summary, my results suggest that pollinators play a minor role in influencing floral adaptation and diversification in Mertensia. Instead, the dominant influences on the traits I examined appear to be life-history trade-offs, environmental conditions that vary along altitudinal gradients, and abiotic variables (e.g., precipitation). It is important to consider these factors and their influences on paternal and maternal fitness in order to gain a broader perspective on floral evolution in plants with generalized pollination systems.

<p>Biodiversity is generated by the process of speciation. Because biological species are defined as populations that are unable to exchange genes with one another, the study of the evolution of reproductive isolation occupies the center of speciation research. A key to deciphering how reproductive isolation evolves is to understand the genetic changes that underlie these barriers to gene flow. Intrinsic postzygotic barriers, such as hybrid sterility or inviability, are known to impede gene flow and especially lend themselves to genetic analysis because of their ease of study in a laboratory setting. Because hybrid sterility likely evolves before hybrid inviability, it potentially plays an important role in the cessation of gene flow. Yet, while their X-linked counterparts have been precisely localized, we remain ignorant of the numbers of and interactions among dominant autosomal loci that are predicted to contribute to F1 hybrid male sterility. </p><p> To address this conceptual void, I examine the genetic architecture of hybrid male sterility between the allopatric sister species Drosophila persimilis and D. pseudoobscura bogotana. First, using a large-scale backcross analysis, I fine-map autosomal QTL from D. persimilis that confer sterility in male hybrids. This fine-mapping shows that loci contributing to hybrid male sterility reside outside chromosomal rearrangements (i.e., regions of reduced recombination) in this allopatric species pairs. In contrast, these QTL do not contribute to hybrid male sterility in the comparable sympatric hybridizing species D. persimilis and D. pseudoobscura, as predicted by models that suggest that hybridizing species persist because of broad regions of reduced recombination. Next, I use a serial backcross design to introgress these sterility-conferring QTL from D. persimilis into a D. p. bogatana genetic background devoid of other alleles from D. persimilis. This introgression study tested a prediction of the dominance theory proposed to explain Haldane's rule: dominant-acting autosomal loci should interact with recessive-acting X-linked loci to produce sterile hybrid males. Surprisingly, the results demonstrated that the "composite" dominance of the autosomal QTL is more important than the dominance of individual QTL for producing Haldane's rule: epistasis among loci elevated their dominant effects on sterility such that individually-recessive-acting autosomal QTL can contribute to F1 male infertility. Finally, using recombination to generate independent lines bearing only small segments of the identified QTL regions, I examine whether single or multiple loci within these regions contribute to the overall effect of hybrid sterility. While the effect of one QTL depends on epistasis between several loci within that small region, the effect of the other QTL appears to derive from a single genetic factor. These results suggest that estimates of the number of genes that contribute to reproductive isolation are at best, likely too low and, at worst, unattainable with the mapping resolution attainable by standard backcross and introgression approaches.</p><p> This dissertation addresses both evolutionary and genetic hypotheses of intrinsic postzygotic isolation. Hybrid male sterility between D. persimilis and D. p. bogotana clearly involves highly specific and complex interactions between homoospecific loci. The mapping results presented here also lay the foundation for the identification and cloning of multiple autosomal sterility-conferring "speciation genes."</p> / Dissertation

Behavioral isolation is an important barrier to gene flow, contributing to the formation and maintenance of animal species. Nevertheless, hybridization occurs more commonly than is generally recognized, occurring in over ten percent of animal species in the wild. Although the genetic consequences of hybridization are of considerable interest given their evolutionary implications, the reasons that animals choose to mate with other species are less clear. I apply mate choice theory to the question of hybridization, using wood warblers (family Parulidae) and swordtail fishes (genus Xiphophorus) as study systems. Over half of the 45 species of North American wood warbler have produced hybrids. Using comparative methods, I address the questions: Do ecological and demographic factors predict hybridization in this family? Similarly, how do phylogeny, song similarity, and sympatry with congeners correlate with hybridization? As with North American wood warblers, behavioral isolation is also considered of primary importance in isolating sympatric species of swordtail fishes. Two species, X. birchmanni and X. malinche, hybridize in several locations in the wild. Through experimentation with these and other Xiphophorus species, I investigate some of the factors that cause female mate choice to vary, possibly contributing to hybridization. Specifically, I address the following questions: Do females become less choosy when predation risk is high, or encounter rates with conspecifics are low? Are female preferences for conspecifics innate, or can they be modified by experience? And, do female preferences for conspecifics vary among species, populations, or experiments? These studies illustrate the utility of treating hybridization as just another possible outcome of variation in mate choice. I find that warbler hybridization correlates with ecological and other variables, that female swordtails become more responsive to heterospecifics when mate choice is costly, and that female preferences for conspecifics are species- and context-dependent. As animal hybridization can have important evolutionary consequences, studying the factors that contribute to this variation can enhance our understanding of the evolutionary process. / text

<p>Adaptation is central to our understanding of the origin of biological diversity. Yet whether adaptive divergence promotes the formation of new lineages remains poorly understood. My dissertation addresses the role of adaptive divergence in diversification and speciation. I also investigate an alternative mechanism: dispersal, which can promote diversification and speciation through its effects on gene flow and allopatry. To address the role of divergent adaptation and dispersal in the process of diversification, I take an integrated approach, combining both comparative methods with quantitative genetics to characterize patterns of diversification and speciation in the tribe Brassiceae and genus Cakile. I start with a comparative study of the role of dispersal and adaptation in diversification, and then focus on the role of climatic and latitudinal divergence in the processes of adaptive divergence and speciation. In general, I find limited evidence for the role of divergent adaptation in the evolution of intrinsic reproductive isolation. Diversification in the tribe Brassiceae appears to be mediated by dispersal ability, while in the genus Cakile, the evolution of intrinsic reproductive isolation is largely independent of ecological divergence. Thus, while divergent adaptation to novel habitats and climate are likely occurring in Brassiceae, mediated in part by the evolution of long-distance dispersal, the evolution of intrinsic genic reproductive barriers appears to not be influenced by adaptation.</p> / Dissertation

Understanding the genetics underlying the speciation process has been a long-standing goal of evolutionary biology. Studying inter-population crosses can elucidate the genetic architecture of reproductive isolation and, ultimately, the process of speciation. Hybridization between two species is often maladaptive and results in offspring with decreased fitness compared to the parental forms. Recently, with the development of molecular and genomic tools, it has become possible to understand how and when reproductive isolation arises and what are the underlying mechanisms in the evolution of genetic incompatibilities. Heliconius is a genus of neotropical butterfly best know for their Müllerian mimicry. Here I focus on Heliconius cydno and Heliconius melpomene, two hybridising sympatric species with low levels of inter-specific hybridisation that nonetheless results in genome-wide signatures of admixture. I show that hybrids develop ovarian tissue and, occasionally, oocytes; and use genomic approaches to examine several potential mechanisms underlying post-zygotic isolation between H. cydno and H. melpomene. Firstly, I investigate evolution by gene duplication and identify loci putatively under divergent selection that may play a role in species divergence and speciation. Secondly, I quantify sexually dimorphic expression in H. melpomene, and calculate rates of molecular evolution between H. melpomene and H. erato. Thirdly, I identify differentially expressed genes in the H. cydno x H. melpomene F1 hybrids that may be involved in the species barrier. Finally, investigate whether epigenetic silencing mechanisms could underlie post-zygotic isolation between H. cydno and H. melpomene by quantifying transposable element expression and small RNAs. Overall, I identify loci that merit further investigation for their potential in maintaining reproductive barriers between these two species. I show that different regions of the genome evolve at different molecular rates but there is no faster-Z effect, and consider how might this affect evolution of reproductive isolation. Finally, I show that aberrant epigenetic silencing, a mechanism behind hybrid sterility that is common in other species, is not correlated with post-zygotic isolation between H. cydno and H. melpomene.

Speciationisusuallyaslowprocessoccurringoverthousandstomillionsofyears.Thismakes speciation research difficult because no direct observation or manipulation is possible. At best, we can gain some insight by inferring the population history and structure in very fine detail by investigating genetic markers in multiple individuals of the nascent species. Today, speciationresearchisinanunprecedentedpositionthankstotheadventofhigh-throughput sequencingmethods,whichmakeiteasier and cheaper than ever before to evaluate multiple markers in many individuals. Speciation is not a straightforward process that happens in the same way every time, but rather a phenomenon occurring when genetic and ecological circumstancesactinginsymphonyultimatelyleadtoreproductiveisolationoftwosubpopula- tions. This is why it is important to study multiple model systems to understand the general principles behind speciation. We worked with two species of nightingales (Luscinia luscinia andL.megarhynchos)thatdivergedapproximately1.8Mya,likelyduetoglacialfluctuations in Europe. Our main goal was to use these new high-throughput sequencing methods to (1) detect interspecific hybrids between the species, (2) estimate levels of interspecific gene flow,(3)findareasofthenightingalegenomethatunderliereproductiveisolationand,finally, (4)...