No description available.

No description available.

Individuals of Eriogonum abertianum Torr. (Polygonaceae) flower in spring, or following onset of summer rains, or both. Within populations flowering time is mainly environmentally determined: there is little genetic variance for flowering time, and experimental moisture limitation significantly delays flowering. In the field a Sonoran Desert population experienced significantly more mortality during the foresummer droughts, and had a significantly greater proportion of spring-flowering plants, than a Chihuahuan Desert population. Greenhouse experiments suggest a genetic basis for differences in size and time of flowering between these populations. Fossil and biogeographic evidence support an adaptive interpretation of earlier flowering in the Sonoran Desert. A model of selection comparing spring-plus-summer flowering with spring-only flowering suggests that expected summer fecundity may not offset the risk of foresummer mortality in the Sonoran population. Rather than switching to a spring-only habit as predicted by the model, the species' range ends where summer rainfall declines abruptly. The invariance of the spring-plus-summer habit is not explained by the demographic, historical, or genetic data. Plants which live for more than a year in the wild have offspring which, in the greenhouse, live longer than the offspring of the general population. This suggests a genetic basis for the occasional observed perennation. Analysis of a quantitative genetic model suggests that when adult survivorship is low, selection will generally reduce perennation. The annual habit is thus likely to persist even in the presence of genetic variation for perennation. Optimal control models of plant carbon allocation are extended to include within-season mortality and allometric growth constraints. When parameters are varied in numerical experiments, resulting predictions for easily measurable characters (e.g., time to first flower) often vary only slightly; most differences are in fitness, suggesting that satisfactory empirical tests may be difficult to conduct. Arbitrary mortality functions can optimally lead to multiple flowering episodes, and this can depend sensitively on parameter values. Optimal trajectories with allometric constraints are divided into a period of vegetative growth and another period of mixed growth.

Thesis by publication. / Thesis (PhD) -- Macquarie University, School of Biological Sciences, 1990. / Bibliography: leaves 269-324. / Introduction -- Models of parental allocation -- Sex expression in homosporous pteridophytes -- The origin of heterospory -- Pollination and the origin of the seed habit -- Brood reduction in gymnosperms -- Pollination: costs and consequences -- Adaptive explanations for the rise of the angiosperms -- Parent-specific gene expression and the triploid endosperm -- New perspectives on the angiosperm female gametophyte -- Overview -- Glossary -- Kinship terms in plants -- Literature Cited. / Among vascular plants/ different life cycles are associated with characteristic ranges of propagule size. In the modern flora, isospores of homosporous pteridophytes are almost all smaller than 150 urn diameter, megaspores of heterosporous pteridophytes fall in the range 100-1000 urn diameter, gymnosperm seeds are possibly all larger than the largest megaspores, but the smallest angiosperm seeds are of comparable size to large isospores. -- Propagule size is one of the most important features of a sporophyte's reproductive strategy. Roughly speaking, larger propagules have larger food reserves, and a greater probability of successful establishment, than smaller propagules, but a sporophyte can produce more smaller propagules from the same quantity of resources. Different species have adopted very different size-versus-number compromises. The characteristic ranges of propagule size, in each of the major groups of vascular plants, suggest that some life cycles are incompatible with particular size-versus-number compromises. -- Sex expression in homosporous plants is a property of gametophytes (homosporous sporophytes are essentially asexual). Gametophytes should produce either eggs or sperm depending on which course of action gives the greatest chance of reproductive success. A maternal gametophyte must contribute much greater resources to a young sporophyte than the paternal gametophyte. Therefore, smaller gametophytes should tend to reproduce as males, and gametophytes with abundant resources should tend to reproduce as females. Consistent with these predictions, large female gametophytes release substances (antheridiogens) which induce smaller neighbouring ametophytes to produce sperm. -- The mechanism of sex determination in heterosporous species appears to be fundamentally different. Large megaspores develop into female gametophytes, and small icrospores develop into male gametophytes. Sex expression appears to be determined by the sporophyte generation. This is misleading. As argued above, the optimal sex expression of a homosporous gametophyte is influenced by its access to resources. This is determined by (1) the quantity of food reserves in its spore and (2) the quantity of resources accumulated by the gametophyte's own activities. If a sporophyte produced spores of two sizes, gametophytes developing from the larger spores' would be more likely to reproduce as females than gametophytes developing from the smaller spores, because the pre-existing mechanisms of sex determination would favor production of archegonia by larger gametophytes. Thus, the predicted mechanisms of sex determination in homosporous species could also explain the differences in sex expression of gametophytes developing from large and small spores in heterosporous species. / Megaspores of living heterosporous pteridophytes contain sufficient resources for female reproduction without photosynthesis by the gametophyte (Platyzoma excepted), whereas microspores only contain sufficient resources for male reproduction. Furthermore, many more microspores are produced than megaspores. A gametophyte's optimal sex expression is overwhelmingly determined by the amount of resources supplied in its spore by the sporophyte, and is little influenced by the particular environmental conditions where the spore lands. Gametophytes determine sex expression in heterosporous species, as well as homosporous species. A satisfactory model for the evolution of heterospory needs to explain under what circumstances sporophytes will benefit from producing spores of two distinct sizes. -- In Chapter 4, I present a model for the origin of heterospory that predicts the existence of a "heterospory threshold". For propagule sizes below the threshold, homosporous reproduction is evolutionarily stable because gametophytes must rely on their own activities to accumulate sufficient resources for successful female reproduction. Whether a gametophyte can accumulate sufficient resources before its competitors is strongly influenced by environmental conditions. Gametophytes benefit from being able to adjust their sex expression in response to these conditions. For propagule sizes above the threshold, homosporous reproduction is evolutionarily unstable, because the propagule's food reserves are more than sufficient for a "male" gametophyte to fertilize all eggs within its neighbourhood. A population of homosporous sporophytes can be invaded by sporophytes that produce a greater number of smaller spores which could land in additional locations and fertilize additional eggs. Such'spores would be male-specialists on account of their size. Therefore, both spore types would be maintained in the population because of frequency-dependent selection. -- The earliest vascular plants were homosporous. Several homosporous groups gave rise to heterosporous lineages, at least one of which was the progeniture of the seed plants. The first heterosporous species appear in the Devonian. During the Devonian, there was a gradual increase in maximum spore size, possibly associated with the evolution of trees and the appearance of the first forests. As the heterospory threshold was approached, the optimal spore size for female reproduction diverged from the optimal spore size for male reproduction. Below the threshold, a compromise spore size gave the highest fitness returns to sporophytes, but above the threshold, sporophytes could attain higher fitness by producing two types of spores. -- The evolution of heterospory had profound consequences. Once a sporophyte produced two types of spores, microspores and megaspores could become specialized for male and female function respectively. The most successful heterosporous lineage (or lineages) is that of the seed plants. The feature that distinguishes seed plants from other heterosporous lineages is pollination, the capture of microspores before, rather than after, propagule dispersal. Traditionally, pollination has been considered to be a major adaptive advance because it frees sexual reproduction from dependence on external fertilization by freeswimming sperm, but pollination has a more important advantage. In heterosporous pteridophytes, a megaspore is provisioned whether or not it will be fertilized whereas seeds are only provisioned if they are pollinated. / The total cost per seed cannot be assessed solely from the seed's energy and nutrient content. Rather, each seed also has an associated supplementary cost of adaptations for pollen capture and of resources committed to ovules that remain unpollinated. The supplementary cost per seed has important consequences for understanding reproductive strategies. First, supplementary costs are expected to be proportionally greater for smaller seeds. Thus, the benefits of decreasing seed size (in order to produce more seeds) are reduced for species with small seeds. This effect may explain minimum seed sizes. Second, supplementary costs are greater for populations at lower density. Thus, there is a minimum density below which a species cannot maintain its numbers. -- By far the most successful group of seed plants in the modern flora are the angiosperms. Two types of evidence suggest that early angiosperms had a lower supplementary cost per seed than contemporary gymnosperms. First, the minimum size of angiosperm seeds was much smaller than the minimum size of gymnosperm seeds. This suggests that angiosperms could produce small seeds more cheaply than could gymnosperms. Second, angiosperm-dominated floras were more speciose than the gymnosperm-dominated floras they replaced. This suggests that the supplementary cost per seed of angiosperms does not increase as rapidly as that of gymnosperms, as population density decreases. In consequence, angiosperms were able to displace gymnosperms from many habitats, because the angiosperms had a lower cost of rarity. -- Angiosperm embryology has a number of distinctive features that may be related to the group's success. In gymnosperms, the nutrient storage tissue of the seed is the female gametophyte. In most angiosperms, this role is taken by the endosperm. Endosperm is initiated by the fertilization of two female gametophyte nuclei by a second sperm that is genetically identical to the sperm which fertilizes the egg. Endosperm has identical genes to its associated embryo, except that there are two copies of maternal genes for every copy of a paternal gene. -- Chapter 9 presents a hypothesis to explain the unusual genetic constitution of endosperm. Paternal genes benefit from their endosperm receiving more resources than the amount which maximizes the fitness of maternal genes, and this conflict is expressed as parent-specific gene expression in endosperm. The effect of the second maternal genome is to increase maternal control of nutrient acquisition. -- Female gametophytes of angiosperms are traditionally classified as monosporic, bisporic or tetrasporic. Bisporic and tetrasporic embryo sacs contain the derivatives of more than one megaspore nucleus. Therefore, there is potential for conflict between the different nuclear types within an embryo sac, but this possibility has not been recognized by plant embryologists. In Chapter 10, I show that many previously inexplicable observations can be understood in terms of genetic conflicts within the embryo sac. / Mode of access: World Wide Web. / 324 leaves ill

Diss. (sammanfattning) Alnarp : Sveriges lantbruksuniversitet, 2004. / Härtill 4 uppsatser.

This study lends strong support to the idea that members of Bromeliaceae have undergone a recent adaptive radiation, and therefore show that, at least in part, diversity in the tropics is due to a fast speciation rate and that the tropics can be a "cradle" for new diversification and exploitation of varying ecological niches through the diversification of ecophysiological traits within a lineage.

No description available.

The genome of living organisms is constantly subjected to the environmental influences that result in different negative, negligible or positive impacts. The ability to maintain the genome integrity and simultaneously provide its flexibility is the main determinant for the evolutionary success of any species. One of the important aspects of genome maintenance is the precise regulation of the DNA repair machinery. Results reported here indicate the existence of a tight, age-dependent regulation of homologous recombination, one of the two main DNA double-strand break repair pathways. We show that recombination is influenced by conditions such as the change of temperature (cold or warm), day length, water availability (drought or overwatering stress) and salinity. These stresses not only influence the genome stability of stress-subjected generations but also change the recombination in subsequent generations. This indicates the possible involvement of homologous recombination in plant evolution and development of plant stress tolerance. / xiv, 121 leaves ; 29 cm.

Despite the existence of various molecular marker systems there are still limitations in distinguishing between closely related species based on molecular divergence, especially when hybridization events have occurred in the past. The characterisation of plant small nucleolar RNA (snoRNA) genes and their organisation into multigene clusters provides a potential nuclear marker system which could help in resolving the phylogenetic history of plants and might be applicable in DNA barcoding. Using closely and distantly related Senecio species, I investigated a combination of fragment length and sequence variation of snoRNA genes/snoRNA gene clusters to assess the utility of this marker system for barcoding and resolving species relationships. SnoRNA gene and gene cluster sequences identified in Arabidopsis thaliana were used to find homologues in other species and subsequently used for the design of universal primers. Most of the universal primer pairs designed were successful in amplifying snoRNA fragments in most Senecio species and fragment length variation between and within species could be detected. Furthermore, the combination of some fragment length datasets produced by different primer pairs enabled the separation of species and the detection of reticulate evolution indicating a high potential of snoRNA gene/gene cluster fragment length polymorphisms (SRFLPs) for phylogenetic reconstructions in Senecio and other plant genera. Most of the examined gene clusters showed a similar gene order in Senecio and Arabidopsis. However, the majority of these clusters appeared to exhibit more copies in Senecio, some of which were distinguishable by a combined sequencing/fragment profiling approach, and shown to be putative single copy regions with the potential to be used as co-dominant markers. However, a high number of paralogues and possible differences in copy number between species excludes these regions from being used in DNA barcoding. This is because specific primers would have to be developed for specific copies which would preclude development of a universal application for barcoding. None of the regions showed enough sequence variation to delimit distinctly closely related Senecio species and were therefore also considered to be unsuitable for DNA barcoding. Although most snoRNA genes and gene clusters might be inapplicable for DNA barcoding, they are likely to be valuable for phylogenetic studies of species groups, genera and families. On this scale, specific primers might act universally and the number of paralogous copies is likely to be equal across the species group of interest.

Pollinators are believed to have played a key role in the radiation of flowering plants. The Grant-Stebbins model of pollinator-mediated speciation, in which evolutionary shifts between pollinators result in phenotypic diversification and enforce reproductive isolation, is one of the most compelling hypotheses for the rapid diversification of angiosperms. A key principle in this model is that plant pollination systems tend towards specialization, resulting in convergent suites of floral traits (syndromes) associated with particular types of pollinators. However, the expectation of pollination system specialization is not always supported by ecological data and has also been questioned on theoretical grounds. In this thesis, I examine pollination by Hemipepsis spider-hunting wasps (Hymenoptera, Pompilidae, Pepsinae) and use this system to address questions about levels and proximal mechanisms of floral specialization, floral shifts and convergent evolution of floral traits. Specialized pollination by Hemipepsis wasps is a newly described pollination system within the angiosperms. I document pollination by these wasps for the first time in 15 South African grassland plant species, including two species of Eucomis (Hyacinthaceae) and 13 asclepiads (Apocynaceae: Asclepiadoideae). In one of the asclepiads, Xysmalobium undulatum, I describe a bimodal pollination system involving both Hemipepsis wasps and a cetoniine beetle. I also describe an unusual and potentially antagonistic pollination mechanism whereby wasps are systematically dismembered during the insertion of pollinia in the two asclepiads Pachycarpus asperifolius and P. appendiculatus. I have used these and previous case studies to establish the existence of a new pollination guild, consisting of at least 21 plant species (across 10 genera and three families), that are reliant on four functionally similar species of Hemipepsis wasp for pollination. Plants in the guild are distributed throughout the moist grasslands of eastern South Africa and flower from September through until early May, peaking in December/January. The Hemipepsis-wasp pollination guild is characterized by high levels of functional specialization (17 of the 21 known guild members are pollinated exclusively by Hemipepsis wasps), despite the absence of morphological adaptations to prevent non-pollinating insects from accessing nectar. I used field and laboratory based experiments to explore the function of floral traits in enforcing specialization. These showed that Hemipepsis wasps primarily use scent, rather than visual cues, to locate flowers, but I was unable to firmly identify specific compounds responsible for the attraction of these wasps (compounds that elicited antennal responses in preliminary GC-EAD experiments did not attract wasps in bioassays). The chemical composition of the floral scents of guild members was examined for 71 individuals representing 14 species in addition to previous studies, and found to comprise complex blends of volatiles (usually containing between 30 and 50 compounds), typically dominated by aliphatics and monoterpenes with small amounts of aromatics. I also showed that the floral colours of guild members are similar to background vegetation, suggesting that floral colours are adapted for crypsis to avoid detection by non-pollinating insects. Palatability choice experiments with honeybees showed that non-pollinating insects find the nectars of at least three of the asclepiad guild members distasteful. Plants in this guild thus appear to achieve specialization through biochemical filters (scent as an attractant and differentially palatable nectar) and cryptic coloration. Pollinator-mediated convergence in floral traits is the fundamental basis for pollination syndromes, but has seldom been rigorously analyzed. Flowers in the Hemipepsis-wasp pollination guild share several qualitative traits, including dull greenish- or brownish-white colour, often with purple blotches, exposed sucrose dominant nectar with a relatively high sugar concentration (typically over 50% sugar by weight) and a sweet/spicy fragrance to the human nose. To test for convergent evolution in guild members, I compared scent, nectar and colour traits of guild members to those of congeners with different pollinators. Although traits often differed between guild members and their congeners, I found little evidence for overall convergence in floral scent profiles and nectar properties, but floral colours in the guild were significantly closer to the colour of background vegetation than those of congeners. At this stage, the lack of knowledge about specific floral volatiles that influence Hemipepsis-wasp behaviour and secondary nectar constituents that limit non-pollinator visits makes it difficult to identify the extent of biochemical convergent evolution within the guild. The directions and functional traits involved in evolutionary transitions between pollination by Hemipepsis wasps and other vectors are currently difficult to ascertain as there is limited phylogenetic data for the plant families concerned. In the genus Eucomis, fly and Hemipepsis-wasp pollinated species are very similar in floral morphology and colour, but differ strongly in floral scent. Using manipulative field experiments in conjunction with detailed analyses of colour, scent and morphology, I was able to show that a shift between wasp and fly pollination could be induced simply by manipulating oligosulphides in the scent emission from inflorescences. When considered in combination with other experiments highlighting the importance of scent as a pollinator attractant for all guild members, this suggests that scent properties may have played a key role in the evolutionary transitions between pollination by Hemipepsis wasps and other vectors. This research has established that pollination by Hemipepsis spider-hunting wasps is more geographically and phylogenetically widespread than was previously known, and has confirmed that these wasps are important and consistent pollinators in southern African grassland ecosystems. I have shown that a distinct guild of plants is specialized for pollination by these wasps. The high levels of specialization within this guild highlight the effectiveness of biochemical filters and cryptic coloration in limiting the spectrum of flower visitors. The major challenge ahead will be to identify the floral volatiles that attract Hemipepsis wasps and the non-sugar constituents that make the nectars of some guild members differentially palatable. These would both contribute greatly to our understanding of floral specialization and the mechanisms involved in the radiation of the angiosperms. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.