Evolution Of Breeding Systems Research Articles

Phylogenetic relationships and character evolution in Primula L.: The usefulness of ITS sequence data

ABSTRACT The main goals of this research were to reconstruct the infrageneric phylogeny of the genus Primula based on both nuclear and chloroplast DNA sequences, and to use the resulting phylogenies to elucidate the evolution of breeding systems, morphological characters, chromosome number, and biogeographic distribution in the genus. In this paper, the results of a pilot study based on the nuclear ribosomal Internal Transcribed Spacer (ITS) region are described. ITS sequences from 21 taxa produced a number of variable characters sufficient to resolve relationships among sections. The resulting phylogeny confirmed the monophyly of sections Auricula and Aleuritia. Sections Armerina, Proliferae, Crystallophlomis, Parryi, and Auricula, with a base chromosome number of x = 11, and sect. Aleuritia, with a base chromosome number of x = 9, formed two well supported clades. The ITS topology also suggested that leaves with revolute vernation, previously believed to be a derived state, might represent the ancestral condition in Primula, with later reversals to the involute condition. Finally, this initial ITS tree provides preliminary support to the proposed role of the widespread, diploid and heterostylous P. mistassinica as having given origin to the polyploid, homostylous P. incana and P. laurentiana.

Using Phylogenetic Approaches for the Analysis of Plant Breeding System Evolution

▪ Abstract Until recently, studies of plant reproductive systems have been at the population level, using microevolutionary approaches. The development of cladistic approaches, combined with the emergence of molecular systematics, has resulted in an explosion of phylogenetic studies and an increase in interdisciplinary approaches combining ecological and systematic methodology. These new approaches offer the possibility of testing explicit hypotheses about the number of evolutionary transitions in reproductive characters and the evolutionary relationship of these characters to changes in the environment. Character mapping may be especially useful for detecting convergent evolution. In a number of cases, character mapping has provided new insights into the evolution of plant breeding systems and pollination biology, especially in suggesting the number of times evolutionary transitions have taken place, indicating where there have been reversals and suggesting when preadaptation has been important. The insights provided by character mapping are determined by a number of factors, including the degree of confidence in phylogenies underlying these studies and the identification of appropriate outgroups. Assumptions about character coding, character ordering, inclusion vs. exclusion of characters that are mapped on trees in the data matrix, and weighting of characters will have profound effects on interpretation of character evolution. Highly labile characters that evolve frequently and have the potential to undergo reversals may make it difficult to detect the pattern of character evolution. Characters that are very strongly correlated with each other or with ecological shifts may make prediction of cause and effect using phylogenetic approaches difficult because changes in characters and ecological shifts will occur, apparently simultaneously, on the same branches. Results from microevolutionary studies have been used in several cases to weight transitions, suggesting that results of phylogenetic studies may not provide fully independent assessments of character evolution. While not a simple cure to understanding problems that have been studied only in the realm of microevolutionary studies, phylogenetic approaches offer clear potential for providing new insights for evolutionary studies.

Evidence for effects of restorer genes on male and female reproductive functions of hermaphrodites in the gynodioecious species Thymus vulgaris L.

Abstract In Thymus vulgaris L., sex determination involves both the nuclear and the cytoplasmic genomes: the cytoplasm is responsible for male-sterility (the female phenotype) while specific nuclear genes may restore male fertility (the hermaphrodite phenotype). Previous observations have shown high variation among hermaphrodites for pollen and seed production. In order to investigate the origin of this variation, 12 female plants, four from each of three populations, were hand-pollinated with pollen from hermaphrodites from three different paternal populations. The sex-ratio (i.e. the frequency of hermaphrodites) produced and the reproductive functions of these offspring were measured. A strong positive correlation was observed between the sex-ratio within a family and both female and male reproductive functions of its hermaphrodites. No such correlation was found for females. This result suggests that restorer genes may be directly or indirectly involved both in sex determination and in the efficiency of resource allocation to reproductive functions. As a consequence, female advantage, i.e. the relative fecundity of females to hermaphrodites, is larger in families with low sex-ratio, and this might affect the evolution of this gynodioecious breeding system.

On the evolutionary pathway of parental care in mouth–brooding cichlid fishes

Evolutionary theory predicts that differences in parental care patterns among species arose from interspecific differences in the costs and benefits of care for each sex. In Galilee St Peter9s fish, Sarotherodon galilaeus (Cichlidae), male care, female care and biparental care all occur in the same population. We exploit this unusual variability to isolate conditions favouring biparental versus uniparental mouth–brooding by males or females. We first review a game–theoretic model of parental care evolution, predictions of which we test experimentally in this paper. Manipulations of the operational sex ratio show that males and females desert their offspring more frequently when the costs of care are high (in terms of lost mating opportunities). Breeding trials with males of different sizes show that small fathers desert more frequently than large fathers. We attribute this to the associated difference in the fitness benefit of biparental care relative to female–only care. Our experimental results confirm that in St Peter9s fish the probability of caring is determined facultatively according to current conditions at each spawn. The experiments and model together suggest that interspecific variation in remating opportunities and clutch size may be responsible for differences in care patterns within the sub–family Tilapiini. Our results support the hypothesis that biparental mouth–brooding was the ancestral state of both male and female uniparental mouth–brooding in cichlid fishes.

Dioecy and the evolution of pollination systems in Schiedea and Alsinidendron (Caryophyllaceae: Alsinoideae) in the Hawaiian Islands

The transition from biotic to wind pollination and the consequencesof this transition for the evolution of dioecious breeding systems wereinvestigated in Schiedea and Alsinidendron(Caryophyllaceae: Alsinoideae), genera endemic to the Hawaiian Islands. The potential for wind pollination was studied for five species ofSchiedea using a wind tunnel. Morphological correlates of windpollination for these species were then used to infer the presence orabsence of wind pollination in the remaining Schiedea species. Hermaphroditic Alsinidendron and Schiedea species,which occur in mesic to wet forests, or less commonly in dry habitats,show little or no indication of wind pollination. These species had lowpollen:ovule ratios, large relative pollen size, diffuse inflorescences,substantial nectar production in several cases, and appear to bebiotically pollinated or autogamous. Sexually dimorphic species, whichall occur in dry habitats, are wind pollinated, based on wind tunnelresults or morphological adaptations indicating the potential for windpollination. These adaptations include high pollen:ovule ratios, smallpollen size, moderately to highly condensed inflorescences, and reducednectaries and nectar production. Shifts to wind pollination anddimorphism are strongly correlated in Schiedea, suggesting theclose functional relationship of the twophenomena.

Energy Intake During Reproduction in Captive Common Marmosets ( Callithrix jacchus)

Among haplorhine primates, the highly specialized Callitrichidae (marmosets and tamarins) are expected to have comparatively high reproductive costs, a feature that might be related to the evolution of a cooperative breeding system. Costs of reproduction in captivity were investigated on the basis of changes in energy intake and body weight during pregnancy and lactation in pair-living female and male common marmosets ( Callithrix jacchus). The experimental design had little effect on carrying behavior, food intake, and body weight of adults, but a negative transitory effect of offspring body weight. Increased energetic requirements during pregnancy did not result in a higher energy intake in females. During lactation, females increased their energy intake up to 100% and gradually lost weight, suggesting even higher costs. Extensive carrying behavior by males, on the other hand, did not result in an increased energy intake in males, or in changes in male body weight. It is suggested that, at least in captivity, increased energetic demands during reproduction are reduced by behavior allocations towards energetically less expensive behaviors.

Life history of the long‐lived gynodioecious cushion plant Silene acaulis (Caryophyllaceae), inferred from size‐based population projection matrices

Alpine plants often appear to have long life-spans as an adaptation to harsh and unpredictable environmental conditions, yet many lack reliable indicators of age that would make it possible to determine their true longevity. Their extended life-spans also pose problems for measuring lifetime reproductive success, a key component of breeding system evolution in species such as the gynodioecious cushion plant Silene acaulis. For a population of S. acaulis in south-central Alaska, we applied a recently derived analytical approach using size-based population projection matrices that allowed us to estimate: (1) the relationship between cushion diameter and age; and (2) lifetime reproductive success through seed production by females relative to hermaphrodites. Because of a combination of slow growth, frequent shrinkage, and extremely high adult survival, we estimate that the largest cushions in our study population exceed 300 yr in age, and some may live substantially longer, despite the seemingly inhospitable alpine environment they inhabit. Females are estimated to produce 4.4 times as many offspring via seed production over the course of their lives as do hermaphrodites, a difference that is more than sufficient to assure the persistence of females despite their inability to transmit genes through pollen. These results highlight the utility of size-based projection matrices for studying the life histories of herbaceous perennials whose life-span and lifetime reproductive success cannot be determined easily by any other means.

Datiscaceae Revisited: Monophyly and the Sequence of Breeding System Evolution

Previous studies of the small angiosperm family Datiscaceae have drawn contradictory con- clusions regarding its monophyly. Clarification of the relationships among the family components is critical to the interpretation of breeding system evolution within this family. Datisca glomerata is the only androdioe- cious member of the otherwise dioecious family and an initial phylogenetic study suggested that this rare breeding system was derived from dioecy in this family. A subsequent, broader scope phylogenetic analysis of Datiscaceae and related families has since suggested that Datiscaceae are not monophyletic, calling into question earlier conclusions regarding the evolution of androdioecy in Datiscaceae. In the present study, the phylogenetic relationships of Datiscaceae and the sequence of breeding system evolution are reexamined. DNA sequences from three sources including nuclear 18S ribosomal DNA, the internal transcribed spacer (ITS) region of nuclear ribosomal DNA, and the chloroplast-encoded rbcL gene were analyzed phylogeneti- cally using parsimony. Results from analysis of rbcL, 18S, and a combined data set all agree that Datiscaceae do not form a monophyletic assemblage. Datisca appears as a sister group to Begoniaceae in all analyses, but the position of sister taxa Octomeles and Tetrameles relative to Datisca and other members of the Cucurbitales is unresolved. The two species of Datisca form separate monophyletic lineages according to ITS analysis, pro- viding no evidence for a progenitor-derivative relationship for the two species. Phylogenetic trees from analy- ses of rbcL and 18S disagree as to whether dioecy or monoecy is ancestral to Datisca, and thus provide no evi- dence as to which sexual system gave rise to androdioecy in D. glomerata, however, there is no evidence for

FITNESS CONSEQUENCES OF MATERNAL AND NONMATERNAL COMPONENTS OF INBREEDING IN THE GYNODIOECIOUS PHACELIA DUBIA.

I assessed the relationship between the level of inbreeding, F, and fitness, and the effects of nonmaternal and maternal components of inbreeding on fitness in Phacelia dubia. I conducted two generations of controlled crosses and tested the performance of the F2 progeny in field and artificial conditions covering the whole life cycle. Inbreeding significantly decreased the individual contribution of seeds to the next generation in the field, but this decrease apparently is not enough to explain the maintenance of gynodioecy. The inbred progeny contributes significantly to the population genetic structure of P. dubia. Fitness estimates and fitness components tended to decrease, usually monotonically, with F. However, nonmonotonic relationships were found in male fitness components and, in some families, in fitness estimates, seed production per fruit, and establishment. Most of the inbreeding depression takes place at the level of seed establishment in the field, but, in artificial conditions the effects of inbreeding were similar at fecundity and establishment. I studied maternal and nonmaternal components of inbreeding by testing the effects of the relatedness of maternal grandparents and parents on the performance of the progeny. Both components affected fitness. Inbreeding depression was conditioned by the level of inbreeding of the maternal plant, but this interaction varied at different fitness components. Also, the magnitude and even the direction of the relationship between fitness and F changed as a result of the combined effects of maternal and nonmaternal components of inbreeding. Such interactions can render convex or concave fitness functions, giving in the latter case the appearance of a false purging. Maternal effects of inbreeding can result from several processes: maternal investment perhaps with serial adjustments during seed development, purging of recessive deleterious genes, and nucleocytoplasmic interactions. These results illustrate the importance of maternal effects of inbreeding, and the complex effects of inbreeding on fitness. A full understanding of the fitness consequences of inbreeding and, therefore, their potential implications in the evolution of breeding systems, should take into account male and female components as well as transgenerational effects in the context of the particular environment in which fitness is evaluated.

Male-biased sex ratio and promiscuous pollination in the dioecious island treeLaurus azorica (Lauraceae)

Pollination ofLaurus azorica (Lauraceae), a dioecious Macaronesian tree, was studied. Male and female trees had the same size distribution. The population had 2.5 times as many male trees as females. In addition, males produced more flowers, and their inflorescences lasted longer. Individual flower lifetime and length of flowering season were the same in both sexes. Between the years of observation, one tree changed sex. Pollinators wereHalictinae bees and the flyTachina canariensis. The bees collected pollen and nectar and the fly collected nectar from both sexes. Both species visited other plants as well. The evolution of breeding systems inLauraceae is discussed.

Distance‐dependent performance of asexual progeny in Allium vineale (Liliaceae)

Local adaptation within and among populations may have an impact on processes ranging from speciation to the evolution of mixed breeding systems and dispersal strategies. It is also one potential factor that could favor the production of asexual over sexual propagules. This field experiment tested whether asexually produced bulbils of Allium vineale demonstrate local adaptation to the parental microsite at the scale of natural dispersal from the parent (5, 25, 50, 100, and 1000 cm). Both "home'' and randomly chosen "away'' genotypes were planted at each location to determine the relative performance of the "home'' genotype. Overall, bulbil performance declined with distance from the parent. In particular, "home'' bulbils outperformed "away'' bulbils at a distance of 25 cm from the parent, indicating that local adaptation has occurred at the scale of natural dispersal in this species. The variance in propagule performance also increased at farther distances from the parent, indicating that the predictability of offspring performance decreases with distance. Fine-scale local adaptation within the range of seed dispersal in this population may be one factor favoring asexual reproduction in Allium vineale.

Phenology and Reproductive Biology of Syntrichia inermis (Bryopsida, Pottiaceae) in the Mojave Desert

Six populations of Syntrichia inermis (Brid.) Hueb. from the Nevada Mojave Desert were sampled every two weeks for a period of one year. Stem elongation rates were very low (0.14 mm yr. -'), with vegetative growth occurring during the winter. Plants were gonioautoicous, with >90% of the plants morphologically bisexual; however, functionally female stems outnumbered functionally male stems. Archegonia were initiated and became receptive in the same winter, whereas antheridia were initiated in the winter and took from one to several years to reach maturity. On an inflorescence basis, the number of archegonia was fewer and less variable than the number of antheridia, with the abortive rate 3-4% for both gametangia. Fertilization occurred during winterlearly spring, with 50% of the current cycle perichaetia becoming fertilized. Embryos were essentially dormant from spring through fall, resuming growth the following winter, when seta elongation and capsule expansion occurred. A protracted late operculum intact phenophase occurred, with spore dispersal beginning in late summerlearly fall. Previously unknown phenological/reproductive features reported for S. inermis include the lowest known rates of stem elongation, gametophytic and sporophytic growth restricted to winter, the longest period of antheridial maturation, a new pattern of antheridial maturation, and a sporophyte maturation pattern that includes 18 + months of dormancy. Phenological studies of mosses began with broad surveys of local floras (e.g., Grimme 1903) and progressed to detailed studies of single species (e.g., Longton & Green 1969). Perhaps the most significant breakthrough in this field occurred when the stages in the life cycle were defined and quantified (Greene 1960). Since then, the dynamics of vegetative growth, gametangial maturation, stem architecture, and sporophyte life history have been presented for a variety of species (e.g., Deguchi & Takeda 1986; Hancock & Brassard 1974; Imura & Iwatsuki 1989; Longton 1979; Miles et al. 1989; Stark 1986; see review in Longton 1990). Such studies often provide data relevant to the evolution of breeding systems, the allocation of resources to sexual reproduction, and the division of effort between sexual and asexual reproduction. The life history features of desert mosses are not well known (Longton 1988; Scott 1982). It is sometimes assumed that bryophytes in the desert are primarily annuals, escaping the heat and dryness by contracting the life cycle into a few months. However, studies on the desiccation resistance of species in the Syntrichia ruralis (Hedw.) Web. & Mohr complex indicated that some species are perennial and able to withstand desiccated conditions for long periods of time without incurring irreparable cellular damage (Bewley 1979; Oliver et al. 1993). Nevertheless, photosynthesis is restricted to periods when the gametophyte is hydrated (Alpert 1979), which are short and intermittent in the American deserts. The phenology of two species in the Pottiaceae has been studied from the Chihuahuan Desert of New Mexico, S. ruralis and Trichostomum sweetii (Bartr.) Stark (as T. perligulatum (Flow.) Zand.). Both of these species were found to be perennial, but differed from one another in several respects. Gametangia were matured in the fall in T. sweetii, vs. the summer for S. ruralis (Mishler & Oliver 1991; Stark & Castetter 1995). In addition, annual stem elongation rates were much higher in S. ruralis, the populations of which were all female and at a higher elevation. Syntrichia inermis [= Tortula inermis (Brid.) Mont.] is known from throughout the American southwest and adjacent Mexico (Flowers 1973; Mishler 1994). This species is common at middle to low elevations in the Mojave Desert, and is a dominant species on soil below 760 m. It is a distinctive species, recognized by the coiled leaves when dry (into a sickle-shape with half the lamina lying horizontal; Weber 1973), autoicous condition, leaf margins recurved from base to apex, leaf apices mucronate, and sporophytes nearly always present (for illustrations see Flowers 1973, p. 203; Mishler 1994, p. 329). When dry, gametophytic plants of S. inermis resemble those of Tortula atrovirens (Sm.) Lindb. [= Desmatodon convolutus 0007-2745/97/13-27$1.6510 This content downloaded from 207.46.13.128 on Tue, 06 Sep 2016 05:44:12 UTC All use subject to http://about.jstor.org/terms 14 THE BRYOLOGIST [VOL. 100

Estimation of deleterious-mutation parameters in natural populations.

The rate and average effects of spontaneous deleterious mutations are important determinants of the evolution of breeding systems and of the vulnerability of small populations to extinction. Nevertheless, few attempts have been made to estimate the properties of such mutations, and those studies that have been performed have been extremely labor intensive, relying on long-term, laboratory mutation-accumulation experiments. We present an alternative to the latter approach. For populations in which the genetic variance for fitness is a consequence of selection-mutation balance, the mean fitness and genetic variance of fitness in outbred generations can be expressed as simple functions of the genomic mutation rate, average homozygous effect and average dominance coefficient of new mutations. Using empirical estimates for the mean and genetic variance of fitness, these expressions can then be solved to obtain joint estimates of the deleterious-mutation parameters. We employ computer simulations to evaluate the degree of bias of the estimators and present some general recommendations on the application of the technique. Our procedures provide some hope for obtaining estimates of the properties of deleterious mutations from a wide phylogenetic range of species as well as a mechanism for testing the validity of alternative models for the maintenance of genetic variance for fitness.

Molecular Phylogenetic Analysis of the Hawaiian Endemics Schiedea and Alsinidendron (Caryophyllaceae)

Schiedea and Alsinidendron (Caryophyllaceae), which represent the fifth or sixth largest endemic radiation of species in the angiosperm flora of the Hawaiian Islands, exhibit striking diversity in morphology, breeding system, and habitat. To gain a historical perspective on this diversity, we conducted a phylogenetic analysis using restriction site variation in chloroplast DNA and nuclear ribosomal DNA. In addition, we compared, and ultimately combined, the molecular data with a recently published morphologi- cal data set. Within the Schiedea-Alsinidendron lineage, DNA variation is limited, and relationships are generally poorly resolved. These results raise the possibility that, following the initial colonization of the Hawaiian archipelago and the early diversification of the complex, much of the complex radiated rapidly and relatively recently. Phylogenetic analyses of DNA data revealed three clades within the complex (the S. membranacea, S. nuttallii, and S. adamantis clades), in agreement with results of a morphologically-based analysis. Molecular data do not, however, support the S. globosa clade, a weakly-supported clade based on morphology. A combined analysis of morphological and molecular data provided both greater resolution and stronger internal support than either data set did individually. The molecular and combined topologies suggest nearly identical patterns of the evolution of sexual dimorphism, habitat shifts, and biogeography within the complex. However, the greater resolution in trees derived from the combined analysis suggests simpler patterns of breeding-system evolution and habitat shifts. Sexual dimorphism may have evolved twice in the complex, with a single reversal to hermaphroditism in one species, or perhaps only once, with three reversals to hermaphroditism. Although the habitat occupied by the ancestor of the complex remains uncertain, it appears that a single shift to dry habitats more or less accompanied the evolution of dimorphic breeding systems, followed by a single shift back to a mesic environment in one species. Alternatively, two parallel shifts to dry habitats may have occurred. Molecular data are consistent with an origin on Kaua'i of the S. membranacea, S. adamantis, and S. nuttallii clades, as suggested by previous morphological analyses. However, both the molecular and combined trees suggest it is equally likely that the complex originated on O'ahu.

Breeding System Evolution in Leavenworthia. II. Genetic and Nongenetic Parental Effects on Reproductive Success in Selfing and More Outcrossing Populations of Leavenworthia crassa

Ovule fertilization, seed abortion, individual seed weight, seed number per fruit, and fruit set were studied in selfing and outcrossing populations of Leavenworthia crassa by conducting multiple small (four- to six-parent) diallel crosses in growth chambers and in the field for one population. I used maximum-likelihood quadratic analysis to partition variance into main and interaction effects, and factorial analysis to examine mechanisms that might cause nonrandom reproductive success. I found little evidence of additive genetic variation in reproductive traits in the families studied; however, this analysis is likely to be conservative, may underestimate genetic variation in gender-specific parental traits (e.g., maternal seed-filling ability or pollen-specific gene activity), and may not deconfound all genetic and environmental effects. In the outcrossing population, variation in fertilization success among male parents differed across habitats, which indicates that pollen performance may be sensitive to the environment of the paternal plant. Neither self-incompatibility nor outbreeding depression contributed greatly to variation in reproductive success. There was no evidence for maternal discrimination among progeny in either population. The major difference in the pattern of reproductive success in selfing versus more outcrossing populations was in the degree of variation in maternal versus paternal effects, with maternal effects tending to be more significant in the selfing population and variation in paternal effects as large or larger in the outcrossing population.

Estimates of outcrossing rates and of inbreeding depression in a population of the marine angiosperm Zostera marina

Despite the great diversity of pollination and fertilization mechanisms observed in marine plants, little is known about the causes or maintenance of this variation. In this study, I estimated outcrossing rates and levels of inbreeding depression in Zostera marina L. (eelgrass), providing the first empirical test of hypotheses about the evolution of breeding systems in plants with submerged flowers. This study also addressed temporal separation of female and male flowering (dichogamy) in eelgrass as a mechanism promoting an outcrossing mating system, and whether the mating system in eelgrass is related to the degree of dichogamy in the field. Outcrossing rates (0<t<1) estimated from two polymorphic allozyme loci indicate that the Z. marina population in intertidal and subtidal habitats in False Bay, Washington, USA, was highly outcrossing in both 1991 (intertidal t=0.905, subtidal t=1.0) and 1992 (intertidal t=0.775, subtidal t=0.611). The outcrossing rates were positively associated with the degree of dichogamy in 1992; intertidal plants exhibited a greater temporal separation of female and male flowering and a higher outcrossing rate than did subtidal plants. Inbreeding depression at seed set was estimated from hand pollinations (self- and outcross) on 20 reproductive individuals from the False Bay population. Averaged across all maternal parents, a greater proportion of outcrossed flowers set seed than selfed flowers; i.e., inbreeding depression was detected. Plants exhibited genetic variation for inbreeding depression, detected as a significant pollination treatment × maternal family interaction in a log-likelihood analysis. By the end of the seed-maturation period (7 mo after intial seed set) some families showed outbreeding depression, i.e., greater fitness in progeny derived from selfing than in progeny from outcrossing. The inbreeding depression in the False Bay population may be an important selective factor contributing to the maintenance of dichogamy and an outcrossing mating system, as proposed for aquatic plants.

Thinning Reduces the Effect of Rust Infection on Jewelweed (Impatiens Capensis)

We examined the relationship between plant density and the probability of infection by a rust (Puccinia recondita) in a natural population of jewelweed (Impatiens capensis). We also reduced plant density in field manipulations to determine whether there is any synergism between intraspecific competition and infection by the rust. The results showed no relationship between plant density and the probability of infection. However, we found that the effect of infection depends on plant density. Uninfected plants grew significantly faster than infected plants under natural conditions of high density, but infection had no significant effect on growth where density was experimentally reduced. Hence, the effect of disease on jewelweed depends on the plant's competitive environment. Such interactions between competition and disease are likely to be important to the population biology and breeding—system evolution of plants.

Interval mapping of viability loci causing heterosis in Arabidopsis.

The genetic basis of heterosis has implications for many problems in genetics and evolution. Heterosis and inbreeding depression affect human genetic diseases, maintenance of genetic variation, evolution of breeding systems, agricultural productivity, and conservation biology. Despite decades of theoretical and empirical studies, the genetic basis of heterosis has remained unclear. I mapped viability loci contributing to heterosis in Arabidopsis. An overdominant factor with large effects on viability mapped to a short interval on chromosome I. Homozygotes had 50% lower viability than heterozygotes in this chromosomal region. Statistical analysis of viability data in this cross indicates that observed viability heterosis is better explained by functional overdominance than by pseudo-overdominance. Overdominance sometimes may be an important cause of hybrid vigor, especially in habitually inbreeding species. Finally, I developed a maximum likelihood interval mapping procedure that can be used to examine chromosomal regions showing segregation distortion or viability selection.

Gender Specialization Across a Climatic Gradient: Experimental Comparison of Monoecious and Dioecious Ecballium

The ecological basis for breeding—system evolution was explored in Ecballium elaterium (L.) A. Rich. (Cucurbitaceae). Two subspecies are parapatric on the Iberian Peninsula: dioicum (dioecious) is found in southern, more arid regions and elaterium (monoecious) is found in northern, wetter regions. Six experimental gardens were set up in three sites that span the climatic gradient of the species' natural range in Spain in order to address two issue: (1) gender specialization in reproductive characters and life history, and (2) ecological specialization of the two subspecies across environments. Overall, the dioecious subspecies outperformed the monoecious subspecies. Monoecious plants, on average, were shorter—lived in all environments. Females of the dioecious subspecies produced similar numbers of female flowers, but more fruits and seeds, than did monoecious plants (cosexuals). The fruits of females contained more, but smaller, seeds than the fruits of cosexuals. Males always produced more male flowers than did cosexuals, disproportionately so with increased plant size (canopy volume); no such disproportionate increase in seed production was found in females. A geographic analysis of gender (proportion of female flowers) in the monoecious plants revealed a striking pattern: dimorphism with increased gender differentiation in drier sites, in contrast to monomorphism with respect to gender in wetter sites. Differences between the subspecies and between habitat types can be related directly to breeding—system differences. The higher overall performance of dioecious plants may be due to enhanced heterozygosity resulting from outcrossing enforced by dioecy. On the other hand, the gender differentiation of the monoecious plants in the drier sites may reflect gender allocation "decisions" enforced by limited resource availability.

INBREEDING DEPRESSION IN MORPHOLOGICAL AND PHYSIOLOGICAL TRAITS OF SCHIEDEA LYDGATEI (CARYOPHYLLACEAE) IN TWO ENVIRONMENTS.

We compared inbreeding depression in hermaphroditic Schiedea lydgatei and its gynodioecious sister species, S. salicaria, to infer the level of inbreeding depression in their common ancestor. With measurements of selfing rates, this information can be used to assess the importance of inbreeding depression in the evolution of breeding systems in S. lydgatei and S. salicaria. Morphological and physiological characters related to fitness were compared for inbred and outcrossed S. lydgatei in high- and low-fertilizer environments in the greenhouse. Seed mass, number of seeds per capsule, germination, survival, biomass, number of flowers, and age at first flowering were compared for inbred versus outcrossed progeny. We also measured inbreeding depression in maximal rates of photosynthetic carbon assimilation and stomatal conductance to water vapor, traits that affect fitness through their influence on plant carbon balance and water-use efficiency (ratio of carbon gain to water loss). All traits except number of seeds per capsule in parents and survival showed inbreeding depression, with the magnitude depending on family and environment. High inbreeding depression is likely in the ancestor of S. lydgatei and S. salicaria, indicating that, with sufficiently high selfing rates, females could spread in populations. Hermaphroditism in S. lydgatei is probably favored by low selfing rates. In contrast, the evolution of gynodioecy in S. salicaria apparently has been favored by relatively high selfing rates in combination with high inbreeding depression.