Mating system and population genetic structure of an androdioecious tree, Fraxinus lanuginosa Koidz. (Oleaceae) in northern Japan

Male fertility advantage within and between seasons in the perennial androdioecious plant Phillyrea angustifolia

BACKGROUND AND AIMS

Androdioecy, the co-occurrence of males and hermaphrodites, is a rare reproductive system. Males can be maintained if they benefit from a higher male fitness than hermaphrodites, referred to as male advantage. Male advantage can emerge from increased fertility owing to resource reallocation. However, empirical studies usually compare sexual phenotypes over a single flowering season, thus ignoring potential cumulative effects over successive seasons in perennials. In this study, we quantify various components of male fertility advantage, both within and between seasons, in the long-lived perennial shrub Phillyrea angustifolia (Oleaceae). Although, owing to a peculiar diallelic self-incompatibility system and female sterility mutation strictly associated with a breakdown of incompatibility, males do not need fertility advantage to persist in this species, this advantage remains an important determinant of their equilibrium frequency.

METHODS

A survey of >1000 full-sib plants allowed us to compare males and hermaphrodites for several components of male fertility. Individuals were characterized for proxies of pollen production and vegetative growth. By analysing maternal progeny, we compared the siring success of males and hermaphrodites. Finally, using a multistate capture-recapture model we assessed, for each sexual morph, how the intensity of flowering in one year impacts next-year growth and reproduction.

KEY RESULTS

Males benefitted from a greater vegetative growth and flowering intensity. Within one season, males sired twice as many seeds as equidistant, compatible hermaphroditic competitors. In addition, males more often maintained intense flowering over successive years. Finally, investment in male reproductive function appeared to differ between the two incompatibility groups of hermaphrodites.

CONCLUSION

Males, by sparing the cost of female reproduction, have a higher flowering frequency and vegetative growth, both of which contribute to male advantage over an individual lifetime. This suggests that studies analysing sexual phenotypes during only single reproductive periods are likely to provide inadequate estimates of male advantage in perennials.

Female sterility associated with increased clonal propagation suggests a unique combination of androdioecy and asexual reproduction in populations of Cardamine amara (Brassicaceae)

BACKGROUND AND AIMS

The coexistence of hermaphrodites and female-sterile individuals, or androdioecy, has been documented in only a handful of plants and animals. This study reports its existence in the plant species Cardamine amara (Brassicaceae), in which female-sterile individuals have shorter pistils than seed-producing hermaphrodites.

METHODS

Morphological analysis, in situ manual pollination, microsatellite genotyping and differential gene expression analysis using Arabidopsis microarrays were used to delimit variation between female-sterile individuals and hermaphrodites.

KEY RESULTS

Female sterility in C. amara appears to be caused by disrupted ovule development. It was associated with a 2.4- to 2.9-fold increase in clonal propagation. This made the pollen number of female-sterile genets more than double that of hermaphrodite genets, which fulfils a condition of co-existence predicted by simple androdioecy theories. When female-sterile individuals were observed in wild androdioecious populations, their ramet frequencies ranged from 5 to 54 %; however, their genet frequencies ranged from 11 to 29 %, which is consistent with the theoretically predicted upper limit of 50 %.

CONCLUSIONS

The results suggest that a combination of sexual reproduction and increased asexual proliferation by female-sterile individuals probably explains the invasion and maintenance of female sterility in otherwise hermaphroditic populations. To our knowledge, this is the first report of the coexistence of female sterility and hermaphrodites in the Brassicaceae.

Insect visitation rates and foraging patterns differ in androdioecious and hermaphrodite-only populations of Laguncularia racemosa (Combretaceae) in Florida

Insect-pollinated Laguncularia racemosa has a variable breeding system; some populations are androdioecious, with male and hermaphroditic plants, while others lack male plants. We observed the foraging behaviours of insects in three androdioecious and three hermaphrodite-only populations of L. racemosa in Florida. In each population, insect visitation rates were estimated from 30–108 timed intervals. We recorded the number of flowers visited by 144–224 insects during foraging bouts made to 15–40 male and hermaphroditic plants. Male plants in androdioecious populations had significantly more visitors than hermaphroditic plants, increasing the number of vectors carrying pollen from male plants. Further, many insects visited few flowers during foraging bouts, which should increase outcrossing frequency. According to mathematical models, male plants benefit from these combined factors. Plants in hermaphrodite-only populations had significantly more visitors than hermaphroditic plants in androdioecious populations. Proportionately more insects visited many flowers during foraging bouts in hermaphrodite-only versus androdioecious populations. The increased likelihood of geitonogamous self-pollination could help explain the lack of male plants in hermaphrodite-only populations. Differences in pollinator assemblages and the relative abundances of several species were responsible for differences in foraging behaviours: Apis mellifera, Bombus sp., Melissodes sp., Xylocopa sp., Euodynerus sp. and a calliphorid species.

DENSITY-DEPENDENT SELF-FERTILIZATION AND MALE VERSUS HERMAPHRODITE SIRING SUCCESS IN AN ANDRODIOECIOUS PLANT

Models of mating-system evolution emphasize the importance of frequency-dependent interactions among mating partners. It is also known that outcross siring success and the selfing rate in self-compatible hermaphrodites can be density dependent. Here, we use array experiments to show that the mating system (i.e., the outcrossing rate) and the siring success of morphs with divergent sex allocation strategies are both density dependent and frequency dependent in androdioecious populations of the wind-pollinated, annual plant Mercurialis annua. In particular, the outcrossing rate is a decreasing function of the mean interplant distance, regulated by a negative exponential pollen fall-off curve. Our results indicate that pollen dispersed from a male inflorescence are over 60% more likely to sire outcrossed progeny than equivalent pollen dispersed from hermaphrodites, likely due to the fact that males, but not hermaphrodites, disperse their pollen from erect inflorescence stalks. Because of this difference, and because males of M. annua produce much more pollen than hermaphrodites, the presence of males in the experimental arrays reduced both the selfing rate and the outcross siring success of hermaphrodites. We use our results to infer a density threshold below which males are unable to persist with hermaphrodites but above which they can invade hermaphroditic populations. We discuss our findings in the context of a metapopulation model, in which males can only persist in well-established populations but are excluded from small, sparse populations, for example, in the early stages of colonization.

Temporal cline in a hybrid zone population between Fraxinus excelsior L. and Fraxinus angustifolia Vahl

The two closely related ash species Fraxinus excelsior L. (common ash) and Fraxinus angustifolia Vahl (narrow-leaved ash) have a broad contact zone in France where they hybridize. However, little is known about the local structure of hybrid zone populations and the isolation mechanisms. We assessed the potential effect of floral phenology on the structure of a riparian ash hybrid zone population in central France. The distribution of flowering times was unimodal and lay between the flowering periods of the two species. Using microsatellite markers, we detected isolation by time, which has possibly originated from assortative mating. Multivariate analyses indicated that morphological variation is not distributed at random with respect to flowering times. Spatial autocorrelation analyses showed that temporal and spatial patterns were tightly linked. Interestingly, despite the fact that the population shows isolation by time, neighbourhood size and historical dispersal variance (sigma = 63 m) are similar to those detected in pure stands of F. excelsior where individuals flower rather synchronously and hermaphrodites are not the most frequent sexual type. Trees flowering at intermediate dates, which comprised the majority of the population, produced on average more flowers and fruits. We detected no significant differences in floral parasite infections relative to reproductive timing, although there was a tendency for late flowering trees to suffer from more gall attack. We discuss the impact of temporal variation in fitness traits and their possible role in the maintenance of the hybrid zone.

Evolutionary consequences of gender plasticity in genetically dimorphic breeding systems

A functional view of gender helps evolutionary biologists evaluate the mechanisms underlying breeding-system evolution. Evolutionary pathways from hermaphroditism to dioecy include the intermediate breeding systems of gynodioecy and androdioecy. These pathways start with the invasion of unisexual mutants, females or males, respectively, followed by alteration of the hermaphrodites to allocate more to the sexual function that the unisexuals lack. Eventually, hermaphrodites become unisexual and dioecy has evolved. Some species evolving along these pathways stop short of completing this second step, or even revert back from dioecy. We evaluate the hypothesis that gender plasticity is involved in these transitions to and from dioecy. Evidence from studies of subdioecious species that have evolved along the gynodioecy pathway suggests that gender plasticity occurs and stabilizes subdioecy by lowering the cost of producing seed. Factors influencing species evolving toward androdioecy, or reverting to androdioecy from dioecy, appear to be more varied and include reproductive assurance, herbivory and gender plasticity. In general, gender specialization appears to be favored in resource-poor environments regardless of which pathway is taken to dioecy.

Paternal effects on functional gender account for cryptic dioecy in a perennial plant

Natural selection operates on the mating strategies of hermaphrodites through their functional gender, i.e. their relative success as male versus female parents. Because functional gender will tend to be strongly influenced by sex allocation, it is often estimated in plants by counting seeds and pollen grains. However, a plant's functional gender must also depend on the fate of the seeds and pollen grains it produces. We provide clear evidence of a paternal effect on the functional gender of a plant that is independent of the resources invested in pollen. In the Mediterranean tree Fraxinus ornus, males coexist with hermaphrodites that disperse viable pollen and that sire seeds; the population would thus appear to be functionally androdioecious. However, we found that seedlings sired by hermaphrodites grew significantly less well than those sired by males, suggesting that hermaphrodites may be functionally less male than they seem. The observed 1 : 1 sex ratios in F. ornus, which have hitherto been difficult to explain in the light of the seed-siring ability of hermaphrodites, support our interpretation that this species is cryptically dioecious. Our results underscore the importance of considering progeny quality when estimating gender, and caution against inferring androdioecy on the basis of a siring ability of hermaphrodites alone.

Nuclear microsatellites reveal contrasting patterns of genetic structure between western and southeastern European populations of the common ash (Fraxinus excelsior L.)

To determine extant patterns of population genetic structure in common ash and gain insight into postglacial recolonization processes, we applied multilocus-based Bayesian approaches to data from 36 European populations genotyped at five nuclear microsatellite loci. We identified two contrasting patterns in terms of population genetic structure: (1) a large area from the British Isles to Lithuania throughout central Europe constituted effectively a single deme, whereas (2) strong genetic differentiation occurred over short distances in Sweden and southeastern Europe. Concomitant geographical variation was observed in estimates of allelic richness and genetic diversity, which were lowest in populations from southeastern Europe, that is, in regions close to putative ice age refuges, but high in western and central Europe, that is, in more recently recolonized areas. We suggest that in southeastern Europe, restricted postglacial gene flow caused by a rapid expansion of refuge populations in a mountainous topography is responsible for the observed strong genetic structure. In contrast, admixture of previously differentiated gene pools and high gene flow at the onset of postglacial recolonization of western and central Europe would have homogenized the genetic structure and raised the levels of genetic diversity above values in the refuges.