Andrea Case | The Kent State University Press (original) (raw)

Papers by Andrea Case

Evolution; international journal of organic evolution, Jan 30, 2015

Selection is frequency dependent when an individual's fitness depends on the frequency of its... more Selection is frequency dependent when an individual's fitness depends on the frequency of its phenotype. Frequency-dependent selection should be common in gynodioecious plants, where individuals are female or hermaphroditic; if the fitness of females is limited by the availability of pollen to fertilize their ovules, then they should have higher fitness when rare than when common. To test whether the fitness of females is frequency dependent, we manipulated the sex ratio in arrays of gynodioecious Lobelia siphilitica. To test whether fitness was frequency dependent because of variation in pollen availability, we compared open-pollinated and supplemental hand-pollinated plants. Open-pollinated females produced more seeds when they were rare than when they were common, as expected if fitness is negatively frequency dependent. However, hand-pollinated females also produced more seeds when they were rare, indicating that variation in pollen availability was not the cause of frequenc...

Reproductive Allocation in Plants, 2005

Evolutionary Ecology, 2004

Annals of Botany, 2012

The remarkable diversity of mating patterns and sexual systems in flowering plants has fascinated... more The remarkable diversity of mating patterns and sexual systems in flowering plants has fascinated evolutionary biologists for more than a century. Enduring questions about this topic include why sexual polymorphisms have evolved independently in over 100 plant families, and why proportions of self-and crossfertilization often vary dramatically within and among populations. Important new insights concerning the evolutionary dynamics of plant mating systems have built upon a strong foundation of theoretical models and innovative field and laboratory experiments. However, as the pace of advancement in this field has accelerated, it has become increasingly difficult for researchers to follow developments outside their primary area of research expertise. † Scope In this Viewpoint paper we highlight three important themes that span and integrate different subdisciplines: the changes in morphology, phenology, and physiology that accompany the transition to selfing; the evolutionary consequences of pollen pool diversity in flowering plants; and the evolutionary dynamics of sexual polymorphisms. We also highlight recent developments in molecular techniques that will facilitate more efficient and cost-effective study of mating patterns in large natural populations, research on the dynamics of pollen transport, and investigations on the genetic basis of sexual polymorphisms. This Viewpoint also serves as the introduction to a Special Issue on the Evolution of Plant Mating Systems. The 15 papers in this special issue provide inspiring examples of recent discoveries, and glimpses of exciting developments yet to come.

Trends in Plant Science, 2012

Interactions between cytoplasmic and nuclear genomes have significant evolutionary consequences. ... more Interactions between cytoplasmic and nuclear genomes have significant evolutionary consequences. In angiosperms, the most common cytonuclear interaction is between mitochondrial genes that disrupt pollen production (cytoplasmic male sterility, CMS) and nuclear genes that restore it (nuclear male fertility restorers, Rf). The outcome of CMS/Rf interactions can depend on whether Rf alleles have negative pleiotropic effects on fitness. Although these fitness costs are often considered to be independent of the ecological context, we argue that the effects of Rf alleles on fitness should be context dependent. Thus, measuring the cost of restoration across a range of environments could help explain geographic and phylogenetic variation in the distribution of Rf alleles and the outcome of CMS/Rf interactions.

Oikos, 2009

Populations of gynodioecious species vary in the ratio of female versus hermaphroditic individual... more Populations of gynodioecious species vary in the ratio of female versus hermaphroditic individuals they contain, and many exhibit higher frequencies of females under poor resource conditions. One important factor limiting female frequencies within populations is predicted to be pollen limitation of seed production, caused by either low abundance of pollen donors or insufficient pollen transfer. However, empirical studies measuring variation in pollen limitation with population sex ratios or resource gradients in gynodioecious plants are inconsistent. Part of this inconsistency may be that pollen limitation and its causes are context-dependent. Another possibility is that sex-specific daily flower production and/or sex-biased visitation are more relevant to the likelihood of pollen limitation than sex ratio based on counting individual plants. In this study, we examined context-dependent pollen limitation in gynodioecious/subdioecious Fragaria virginiana. We specifically examined the potential for resource availability to influence sex-specific daily flower production, sex-biased pollinator visitation, and their relationships with pollen limitation in experimental populations that contained either high or low frequencies of female plants. High resource availability reduced apparent female frequency by increasing daily flower production by hermaphrodites relative to females. This is important because pollinators increasingly discriminated against female flowers as floral sex ratios became more female-biased. Contrary to expectation, females in high-female populations were not consistently more pollen limited than those in low-female populations. The level of pollen limitation of females was better explained by sexÁbiased pollinator foraging and visitation frequency than by the plant sex ratio or floral sex ratio. Thus, negative frequency dependence of female pollen limitation was evident only considering sex ratio bias mediated by pollinator visitation.

New Phytologist, 2010

*In many gynodioecious plants, sex is determined by cytoplasmic male sterility genes (CMS) and nu... more *In many gynodioecious plants, sex is determined by cytoplasmic male sterility genes (CMS) and nuclear male fertility restorers (Rf). Models predict that the costs of restoration are important determinants of population sex ratios. However, current approaches to the estimation of these costs require prior identification of CMS genotypes, information that is available for few species. *We tested a novel approach to estimating the cost of restoration in natural populations without determining CMS or Rf genotypes. We used estimates of pollen viability and offspring sex ratios from open- and hand-pollinated families of Lobelia siphilitica to test whether the cost of restoration, expressed as low pollen viability, is higher in populations with more females. *Among populations with CMS, we found that variation in pollen viability was higher in small populations with more females, as expected if the proportion of females within populations increases with the maximum cost of restoration. In controlled crosses, families with low pollen viability also produced fewer females, suggesting that variation in viability is primarily determined by the number and frequency of Rf alleles carried. *This approach to estimating the cost of restoration can be applied to other cytonuclear gynodioecious species, offering new opportunities for testing gynodioecy models in the wild.

Journal of Evolutionary Biology, 2007

Variation in population sex ratio can be influenced by natural selection on alternate sex phenoty... more Variation in population sex ratio can be influenced by natural selection on alternate sex phenotypes as well as nonselective mechanisms, such as genetic drift and founder effects. If natural selection contributes to variation in population sex ratio, then sex ratio should covary with resource availability or herbivory. With nonselective mechanisms, sex ratio should covary with population size. We estimated sex ratio, resource availability, herbivory and size of 53 populations of gynodioecious Lobelia siphilitica. Females were more common in populations with higher annual temperatures, lower soil moisture and lower predation on female fruits, consistent with sex-specific selection. Females were also more common in small populations, consistent with drift, inbreeding or founder effects. However, small populations occurred in areas with higher temperatures than large populations, suggesting that female frequencies in small populations could be caused by sex-specific selection. Both selective and nonselective mechanisms likely affect sex ratio variation in this gynodioecious species.

Journal of Ecology, 1999

0 Populations of subdioecious species contain female\ male and cosexual plants[ Here we investiga... more 0 Populations of subdioecious species contain female\ male and cosexual plants[ Here we investigate factors in~uencing gender expression and patterns of resource allocation among the sex phenotypes in subdioecious populations of the diminutive geophyte Wurmbea dioica "Colchicaceae# from the Australian Capital Territory\ south!east Australia[ 1 Demographic studies and population comparisons of the sex phenotypes were conducted to determine their distinguishing features\ and whether sex expression was labile[ Comparisons of biomass allocation were made to assess whether plants with hermaphrodite~owers were larger than unisexual plants[ The seed fertilities of sex phenotypes were measured and related to the sex ratio of populations[ 2 Female plants exhibited canalized gender producing only ovuliferous~owers\ which were smaller and less showy compared to~owers of other plants[ Some male plants displayed labile gender expression\ producing varying numbers of male and her! maphrodite~owers "fruiting males#[ Maternal investment among male plants was size!dependent[ 3 Fruiting males were signi_cantly larger and produced more~owers than unisexual plants[ At~owering\ we found no evidence for changes in proportional allocation with increases in size after controlling for total biomass[ At fruiting\ allocation pat! terns di}ered between females and fruiting males independently of size[ 4 Total seed output by fruiting males was 69) of that produced by females[ The number of seeds per fruit declined acropetally in both sexes\ and males produced\ on average\ more seeds per fruit than females[ Data on the relation between female frequency and the relative seed fertilities of males and females gave a poor _t to Lloyd|s "0865# genetic model predicting equilibrium female frequency[ The lack of congruence probably occurs because size!dependent gender modi_cation in males complicates the assumptions of the model[ Keywords] allocation strategies\ sex ratios\ sexual dimorphism\ size!dependent gender modi_cation Journal of Ecology "0888# 76\ 012Ð026 S[C[H[ Barrett\

International Journal of Plant Sciences, 2008

Using phylogenies to make sound inferences about character evolution depends on a variety of fact... more Using phylogenies to make sound inferences about character evolution depends on a variety of factors, including tree uncertainty, taxon sampling, and the degree of evolutionary lability in the character of interest. We explore the effect of these and other sources of ambiguity for maximum likelihood (ML)-based inferences of sexual-system evolution in Wurmbea, a small genus of geophytic monocots from the Southern Hemisphere. We reconstructed Wurmbea phylogeny using four noncontiguous regions (ca. 5.5 kb) of the plastid genome across a broad sampling of taxa, and we confirm that the genus is divided into two well-supported clades, each defined by its geography (Africa vs. Australasia) and variation in sexual system (i.e., uniformly monomorphic vs. sexually variable, respectively). We demonstrate that the predominantly Australian clade includes the sexually monomorphic species Iphigenia novae-zelandiae. We observe treewide uncertainty in the state of all ancestral nodes, and therefore all state transitions, when all taxa in Wurmbea are considered. We demonstrate that this is primarily a consequence of interspersion of terminals with gender dimorphism vs. monomorphism throughout the Australasian clade, rather than tree uncertainty or the presence of very short internal branches. We accounted for tree uncertainty by randomly sampling alternative resolutions of branches that are poorly supported by ML bootstrap analysis, effectively interpreting these as soft polytomies. Under the assumption that well-supported aspects of our gene tree accurately depict organismal phylogeny, there is a marked evolutionary lability in the sexual systems of Australasian Wurmbea. A more problematic issue is that our results contradict the monophyly of two sexually polymorphic Australian species, Wurmbea dioica and Wurmbea biglandulosa. If this reflects paraphyly at the species level, lateral gene transfer, or failed coalescence, then the interpretations of character transitions will need to be adjusted. Our analysis provides an example of the impediments to linking macroevolutionary pattern with microevolutionary processes for evolutionarily labile traits in recently evolved plant groups that possess a high degree of variation in sexual characters.

International Journal of Plant Sciences, 2004

We compared the floral biology of female, male, and cosexual plants in sympatric populations of W... more We compared the floral biology of female, male, and cosexual plants in sympatric populations of Wurmbea dioica (Colchicaceae) from Western Australia to assess evidence that changes in pollination biology accompanied a transition between monomorphic to dimorphic sexual systems in this species. In W. dioica, sexspecific differences in floral design and display, the quantity and quality of rewards, and reproductive phenology represent alternative strategies for pollinator attraction and the avoidance of self-pollination. Unisexual plants had smaller flowers containing less pollen than flowers of cosexuals; males and cosexuals produced nectar with higher sugar content than did females. These patterns were associated with differences in the composition and visitation rates of pollinators. Nectar-foraging flies visited all three sexes, but pollencollecting bees visited only cosexuals. Fly-pollinated females achieved greater seed fertility relative to their beepollinated cosexual relatives, and males and cosexuals received longer and more frequent visits than did females. Although there were no differences in flower number among sexes, females maintained larger displays throughout flowering. In contrast, males and cosexuals staggered both floral anthesis and anther dehiscence as mechanisms for promoting enhanced pollen dispersal. We propose that sex-based differences in floral design and display in W. dioica have mediated a shift in the effectiveness of nectar-foraging insects as pollinators and that gender dimorphism enforces outcrossing under pollination conditions that would otherwise cause selfing. Differences in pollination biology probably also contribute to the maintenance of combined versus separate sexes in sympatric populations of W. dioica in Western Australia.

Genome Biology and Evolution, 2012

Despite intense investigation for over 25 years, the in vivo structure of plant mitochondrial gen... more Despite intense investigation for over 25 years, the in vivo structure of plant mitochondrial genomes remains uncertain. Mapping studies and genome sequencing generally produce large circular chromosomes, whereas electrophoretic and microscopic studies typically reveal linear and multibranched molecules. To more fully assess the structure of plant mitochondrial genomes, the complete sequence of the monkeyflower (Mimulus guttatus DC. line IM62) mitochondrial DNA was constructed from a large (35 kb) paired-end shotgun sequencing library to a high depth of coverage (;30Â). The complete genome maps as a 525,671 bp circular molecule and exhibits a fairly conventional set of features including 62 genes (encoding 35 proteins, 24 transfer RNAs, and 3 ribosomal RNAs), 22 introns, 3 large repeats (2.7, 9.6, and 29 kb), and 96 small repeats (40-293 bp). Most paired-end reads (71%) mapped to the consensus sequence at the expected distance and orientation across the entire genome, validating the accuracy of assembly. Another 10% of reads provided clear evidence of alternative genomic conformations due to apparent rearrangements across large repeats. Quantitative assessment of these repeat-spanning read pairs revealed that all large repeat arrangements are present at appreciable frequencies in vivo, although not always in equimolar amounts. The observed stoichiometric differences for some arrangements are inconsistent with a predominant master circular structure for the mitochondrial genome of M. guttatus IM62. Finally, because IM62 contains a cryptic cytoplasmic male sterility (CMS) system, an in silico search for potential CMS genes was undertaken. The three chimeric open reading frames (ORFs) identified in this study, in addition to the previously identified ORFs upstream of the nad6 gene, are the most likely CMS candidate genes in this line.

Evolutionary Ecology, 2000

Stressful ecological conditions have been implicated in the evolution of separate sexes in plants... more Stressful ecological conditions have been implicated in the evolution of separate sexes in plants. Gender dimorphic species are often found in drier habitats than their sexually monomorphic relatives, and gynodioecious populations appear closer to a dioecious state as resources, particularly water, become limiting. This pattern could result if dry conditions decrease the relative seed fitness of cosexual plants, allowing female plants to become established in monomorphic populations. We studied geographical variation in gender expression and biomass allocation among 12 monomorphic and dimorphic populations of Wurmbea dioica along a latitudinal precipitation gradient in southwestern Australia to provide insight into mechanisms by which aridity might favor transitions between sexual systems. Plants in monomorphic and dimorphic populations exhibited contrasting gender expression and patterns of biomass allocation in areas with different levels of precipitation. Among dimorphic populations, lower precipitation was associated with a higher frequency of female plants, and reduced allocation to female function by hermaphrodites during flowering. In contrast, stress conditions had no effect on female allocation at flowering in monomorphic populations. Across latitudes, unisexuals and cosexuals exhibited consistent differences in above ground traits, with cosexuals having larger leaves, taller stems and larger flowers. Although all plants were smaller under drier conditions, cosexuals decreased above ground allocation to vegetative and reproductive structures with decreasing latitude. In contrast, unisexuals increased allocation to reproduction in drier areas at the expense of below ground size. Aridity was associated with reduced flower size among all gender classes, but not with changes in flower number. These data do not support the hypothesis that resource limitation of female allocation in cosexual populations favors the establishment of gender dimorphism in W. dioica. Alternative hypotheses, involving higher selfing rates and enhanced survival of unisexuals relative to cosexuals under resource-limited conditions, are discussed as possible explanations for the origin of dioecy in W. dioica.

Evolution, 2007

Resources, sex ratio, and seed production by hermaphrodites covary among natural populations of m... more Resources, sex ratio, and seed production by hermaphrodites covary among natural populations of many gynodioecious plant species, such that they are functionally "more dioecious" as resources become more limiting. Strong correlations among these three factors confound our understanding of their relative roles in maintaining polymorphic sexual systems. We manipulated resource availability and sex ratio and measured their effects on relative fertility and phenotypic selection through the maternal fitness of females and hermaphrodites of Fragaria virginiana. Two results were particularly surprising. First, hermaphrodites showed little variability in fecundity across resource treatments and showed strong positive and context-dependent selection for fruit set.

Evolution, 2008

-sterile phenotypes are rarely expressed in natural populations of angiosperms, CMS genes are tho... more -sterile phenotypes are rarely expressed in natural populations of angiosperms, CMS genes are thought to be common. The evolutionary dynamics of CMS/Rf systems are poorly understood, leaving gaps in our understanding of mechanisms and consequences of cytonuclear interactions. We characterized the molecular basis and geographic distribution of a CMS gene in Mimulus guttatus. We used outcrossing M. guttatus (with CMS and Rf) to self-fertilizing M. nasutus (lacking CMS and Rf) to generate hybrids segregating for CMS. Mitochondrial transcripts containing an essential gene (nad6) were perfectly associated with male sterility. The CMS mitotype was completely absent in M. nasutus, present in all genotypes collected from the original collection site, but in only two individuals from 34 other M. guttatus populations. This pattern suggests that the CMS likely originated at a single locality, spread to fixation within the population, but has not spread to other populations, indicating possible ecological or genetic constraints on dispersal of this CMS mitotype between populations. Extreme localization may be characteristic of CMS in hermaphroditic species, in contrast to geographically widespread mitotypes commonly found in gynodioecious species, and could directly contribute to hybrid incompatibilities in nature.