Patterns of self compatibility, inbreeding depression, outcrossing, and sex allocation in a marine bryozoan suggest the predominating influence of sperm competition

Sex allocation by simultaneous hermaphrodites is theoretically influenced by selfing rate, which is in turn influenced by the benefits of enhanced genomic transmission and reproductive assurance relative to the cost of inbreeding depression. The experimental investigation of these influences in seed plants has a rich pedigree, yet although such an approach is equally relevant to colonial invertebrates, which globally dominate subtidal communities on firm substrata, such studies have been scarce. We reared self‐compatible genets of the marine bryozoan Celleporella hyalina s.l. in the presence and absence of allosperm, and used molecular genetic markers for paternity analysis of progeny to test theoretical predictions that: (1) genets from focal populations with high selfing rates show less inbreeding depression than from focal populations with low selfing rates; (2) genets whose selfed progeny show inbreeding depression prefer outcross sperm (allosperm); and (3) genets bias sex allocation toward female function when reared in reproductive isolation. Offspring survivorship and paternity analysis were used to estimate levels of inbreeding depression and preference for outcrossing or selfing. Sex allocation was assessed by counting male and female zooids. As predicted, inbreeding depression was severe in selfed progeny of genets derived from the populations with low self‐compatibility rates, but, with one exception, was not detected in selfed progeny of genets derived from the populations with higher self‐compatibility rates. Also, as predicted, genets whose selfed progeny showed inbreeding depression preferred outcrossing, and a genet whose selfed progeny did not show inbreeding depression preferred selfing. Contrary to prediction, sex allocation in the majority of genets was not influenced by reproductive isolation. Lack of economy of male function may reflect the over‐riding influence of allosperm‐competition in typically dense breeding populations offering good opportunity for outcrossing. We suggest that hermaphroditism may be a plesiomorphic character of the crown group Bryozoa, prevented by phylogenetic constraint from being replaced by gonochorism and therefore not necessarily adaptive in all extant clades. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 519–531.     

Post‐pollination process in a partially self‐compatible distylous plant,Primula sieboldii (Primulaceae)

Pollen germination and pollen‐tube growth under natural conditions were observed in a population of a distylous species, Primula sieboldii, in which partial self‐compatibility has been demonstrated in some long‐styled genets. We observed post‐pollination processes microscopically in styles collected after self‐morph and inter‐morph hand pollination (with standardized pollen load on the stigmas) in four genets each from the following three ‘genet types’: self‐incompatible long‐styled (SI), partially self‐compatible long‐styled (SC) and self‐incompatible short‐styled morph genets. Irrespective of the genet type, pollen germination began within 24 h after pollination and tubes of pollen reached to the style base with 48–96 h after inter‐morph pollination. Although pollen tubes germinated after self‐pollination in the SC genets, the number of germinated pollen tubes was significantly lower than in the case of inter‐morph pollination. Few pollen tubes germinated after self‐pollination of the SI or short‐styled genets. In SC genets, the rate of pollen‐tube growth did not differ between self‐morph and inter‐morph pollination (～1.9 mm/day). Therefore, differences in self‐compatibility between SC and SI genets in P. sieboldii are likely to be attributable to differential pollen germination rates rather than to differential pollen‐tube growth rates.     

Female and male fitness consequences of clonal growth in a dwarf bamboo population with a high degree of clonal intermingling

Background and Aims

Although many studies have reported that clonal growth interferes with sexual reproduction as a result of geitonogamous self-pollination and inbreeding depression, the mating costs of clonal growth are expected to be reduced when genets are spatially intermingled with others. This study examined how clonal growth affects both female and male reproductive success by studying a population of a mass-flowering plant, Sasa veitchii var. hirsuta, with a high degree of clonal intermingling.

Methods

In a 10 × 10 m plot, genets were discriminated based on the multilocus genotypes of 11 nuclear microsatellite loci. The relationships between genet size and the components of reproductive success were then investigated. Male siring success and female and male selfing rates were assessed using paternity analysis.

Key Results

A total of 111 genets were spatially well intermingled with others. In contrast to previous studies with species forming distinct monoclonal patches, seed production linearly increased with genet size. While male siring success was a decelerating function of genet size, selfing rates were relatively low and not related to genet size.

Conclusions

The results, in conjunction with previous studies, emphasize the role of the spatial arrangement of genets on both the quantity and quality of offpsring, and suggest that an intermingled distribution of genets can reduce the mating costs of clonal growth and enhance overall fitness, particularly female fitness.     

Influence of inbreeding depression on a lake population of Nymphoides peltata after restoration from the soil seed bank

The negative effects of inbreeding depression on fragmented small populations are likely to be expressed more strongly after restoration efforts if regeneration processes have been highly restricted in degraded habitats. We examined the potential influences of inbreeding depression on a population of Nymphoides peltata (Menyanthaceae) restored from the remnant soil seed bank. A hand-pollination experiment demonstrated self-compatibility of a single remaining homostyle genet and significant inbreeding depression in selfed progeny, especially in parameters related to seedling growth (–0.6 for biomass, and –0.4 for relative growth rate). Our genetic analysis indicated that the presumed number of parents contributing to the current soil seed bank was only 2–8 genets and that a single sib-family dominated at each of three sampling sites. The results also showed that the selfed progeny of the homostyle genet were overwhelmingly dominant at two sites (86.8 and 94.7%). As a result, the growth performance of the seed bank seedlings was significantly reduced, to a level as low as that of the selfed progeny. Active restoration efforts to minimize the negative effects of the genetic bottleneck and continuous monitoring based on genetic and demographic study are recommended.     

Testing the 'assortative mating' hypothesis on a variation maintenance mechanism for flowering time within a forest-floor population of Primula sieboldii

Flowering phenology and allozyme variation were studied to test the existence of positive assortative mating for flowering time in a natural population of Primula sieboldii E. Morren, a heterostylous perennial herb, consisting of approximately 180 genets in a deciduous forest. There was significant variation in flowering date among genets, but not between heterostylous morphs. The temporal order of the flowering time of genets was fairly constant for the two years of the study. The spatial heterogeneity of light availability at the study site was small during the flowering season of the species. In order to analyze the extent of genetic differentiation between early- and late-flowering genet groups, allozyme diversities were analyzed with 10 loci. The G_ST between the early- and late-flowering groups was not significantly different from zero. Evidence of positive assortative mating for flowering time was not detected. Prolongation of flowering duration due to pollen limitation may be one important factor preventing the genetic differentiation of early- and late-flowering groups by enhancing the overlap of flowering time among genets.     

GEOGRAPHICAL VARIATION IN THE INBREEDING DEPRESSION OF SCOTS PINE

The magnitude of inbreeding depression caused by recessive mutations in a population is dependent on the mutation rate and on the intensity of selection against the mutations. We studied geographical differences in the level of early inbreeding depression of Scots pine in a common garden experiment. The mean abortion rate of experimentally self-pollinated seeds was significantly lower (75.4%) among trees that originated from northern populations (66–69°N) than among trees from more southern (60–62°N) populations (86.5%). Thus, the number of embryonic lethal equivalents was lower in the northern populations (4.5) than in the southern ones (6.9). The outcrossing rate at the mature seed stage was slightly lower in the northern populations (average 0.93) than in the southern one (0.99). The estimated selfing rate at the zygote stage varied from 0–0.28 in the populations. The reduction in the magnitude of inbreeding depression in the north may have been caused by increased levels of self-fertilization in the northern populations. The proportion of self-fertilized seedlings and adults was very small in all populations (F ≈ 0), indicating high inbreeding depression also in later life stages. The high level of inbreeding depression in the partially selfing Scots pine can be explained by mutation-selection balance only if the mutation rate is high.     

Implications of nonadventitious rhizome spread on reproduction, inbreeding, and conservation for a rare grassland legume

Small population size, genetic diversity, and spatial patterns of vegetative spread are important aspects to consider when managing populations of rare clonal plant species. We used 5 variable nuclear simple sequence repeat nDNA loci to determine the extent of genet rhizome spread, examine the possibility of very small population sizes, and project how Bombus spp. (bumblebee) foraging may impact selfing (through geitonogamy) for a threatened lupine (Lupinus oreganus Heller) that sprawls through nonadventitious rhizomes. Genotyping identified 1 genet (27 × 13 m) that dominated about 30% of a study site, whereas 15 genets spread a maximum average distance of about 5.5 m (range 1.6 -27.1 m) and appeared to be well integrated with intervening genets. We found unexpectedly high genotype diversity, no evidence of a recent genetic bottleneck, and 5 of 6 patches had mean fixation index values that were near Hardy-Weinberg Equilibrium expectations. If the median maximum Bombus foraging distance observed in lupine patches (1.2 m) occurred within genotyped populations, a typical foraging flight would have >80% chance of occurring between different genets. Our study demonstrates that inferences associated with clonality, small population size, and inbreeding depression should be directly evaluated for rare vegetatively spreading plants.     

ECOLOGICAL GENETICS OF GYNODIOECY IN SILENE VULGARIS: RELATIVE FITNESS OF FEMALES AND HERMAPHRODITES DURING THE COLONIZATION PROCESS

Recent theoretical models have addressed the influence of metapopulation dynamics on the fitness of females and hermaphrodites in gynodioecious plants. In particular, selection is thought to favor hermaphrodites during population establishment because that sex should be less prone to pollen limitation, especially if self-fertilization is possible. However, inbreeding depression could limit this advantage. In this experimental study of Silene vulgaris, a weedy gynodioecious plant, the fitness of females and hermaphrodites was estimated from seed production in both mixed-sex populations and for individuals isolated from these populations by 20, 40, 80, or 160 m. In mixed populations females display statistically significant greater per capita seed production owing to higher capsule production and higher rates of seed germination. The fitness of both sexes declines with increasing isolation, but at different rates, such that in the 160-m treatment hermaphrodites are by far the more fit sex. Allozyme studies suggest that this differential decline is because the selfing rate in hermaphrodites increases as a function of isolation, at least partially compensating for a decline in the availability of outcross pollen. Overall, the negative effects of pollen limitation on females far outweighs the negative effects of inbreeding depression following selfing in hermaphrodites. Thus, extinction/recolonization dynamics would appear to favor hermaphrodites as long as seed dispersal events exceed some critical distance.     

Pollen Dispersal Between Isolated Trees in the Brazilian Savannah: A Case Study of the Neotropical Tree Hymenaea stigonocarpa

This case study examines the pollen dispersal distance, pollen dispersal patterns and intra‐family genetic structure for isolated trees in pastures of the bat‐pollinated Neotropical tree species Hymenaea stigonocarpa using six microsatellite loci and parentage analysis. The sampling included 28 grouped trees (referred to as the population) and six isolated trees in pastureland along a highway in Mato Grosso do Sul State, Brazil. From the population, we sampled 137 seeds from 12 seed‐trees, and from the isolated trees, we sampled 34 seeds from two seed‐trees. The results showed that pollen was dispersed over long distances (reaching 7353 m) and therefore the spatially isolated trees were not reproductively isolated. The pollen immigration rate in the population was also high (31%). Isolated trees presented a higher selfing rate (s=26%) than trees in the population (s=12%), suggesting that the spatial isolation of the trees increased selfing. However, selfing was responsible for only 30 percent of the inbreeding in offspring and mating among relatives was 70 percent. In the population, excluding selfing, ca 72 percent of the pollen was dispersed over distances <1000 m (average: 860 m). For the two isolated seed‐trees, excluding selfing, the average pollen dispersal distance was 5229 m. The results demonstrate that although pollen can be dispersed over long distances for H. stigonocarpa isolated trees, a high percentage of pollen comes from the same tree (selfing) and mating was correlated. Consequently, seeds must be collected from a large number of seed‐trees for conservation purposes.     

Selfing and inbreeding depression in seeds and seedlings of Neobalanocarpus heimii (Dipterocarpaceae)

We evaluated the degree of selfing and inbreeding depression at the seed and seedling stages of a threatened tropical canopy tree, Neobalanocarpus heimii, using microsatellite markers. Selection resulted in an overall decrease in the level of surviving selfed progeny from seeds to established seedlings, indicating inbreeding depression during seedling establishment. Mean seed mass of selfed progeny was lower than that of outcrossed progeny. Since the smaller seeds suffered a fitness disadvantage at germination in N. heimii, the reduced seed mass of selfed progeny would be one of the determinants of the observed inbreeding depression during seedling establishment. High selfing rates in some mother trees could be attributed to low local densities of reproductive individuals, thus maintenance of a sufficiently high density of mature N. heimii should facilitate regeneration and conservation of the species.     

THE INFLUENCE OF THE MATING SYSTEM ON SEED SIZE VARIATION IN CRINUM ERUBESCENS (AMARYLLIDACEAE)

We present evidence that extreme seed size variation in fruits of Crinum erubescens (range: 0.1 to 66.5 g per seed) occurs when mating pairs are inbred, either from selfing or biparental inbreeding. Several relatively uniform seeds of intermediate size are produced when pollen from several pollen donors is applied simultaneously to a flower. Selfed fruits and some fruits pollinated with a single pollen donor produce both large and small seeds, although selfed fruits produce fewer seeds than outcrossed fruit. These results are contrary to the hypothesis that variation in seed size is attributable to either pollen competition or differential allocation of maternal resource to seeds of different genotypes.     

The effects of pollen diversity on plant reproduction: insights from apple

For many plants, the number of pollen genotypes deposited on a flower’s stigma is positively related to fruit maturation and seed number; however, the mechanisms underlying this effect are less understood. Here we examined whether diversity of pollen (1, 3, or 5 donors) affects reproductive success in self-incompatible apple (Malus × domestica). Using paternity analysis, we then assessed the siring rate of individual donors to test whether diversity effects are related to the presence of a superior pollen donor or to donor × donor interactions. Increasing the diversity of compatible pollen enhanced seed number and reduced seed abortion in some but not all recipient genotypes. This effect was associated with an increased number of sires per fruit and non-random siring success among pollen donors. Two donors had consistently high siring rates, regardless of recipient genotype; however, the siring success of pollen donors was not correlated with their siring success in single-donor pollinations. Rather, siring success was affected by the identity of other pollen genotypes present on the stigma. Our results therefore suggest that the effects of pollen diversity are variable but may enhance fecundity by fostering interactions between pollen donors.     

Effects of population size and metapopulation dynamics on a mating-system polymorphism

The evolutionary dynamics of neutral alleles under the Wright-Fisher model are well understood. Similarly, the effect of population turnover on neutral genetic diversity in a metapopulation has attracted recent attention in theoretical studies. Here we present the results of computer simulations of a simple model that considers the effects of finite population size and metapopulation dynamics on a mating-system polymorphism involving selfing and outcrossing morphs. The details of the model are based on empirical data from dimorphic populations of the annual plant Eichhornia paniculata, but the results are also of relevance to species with density-dependent selfing rates in general. In our model, the prior selfing rate is determined by two alleles segregating at a single diploid locus. After prior selfing occurs, some remaining ovules are selfed through competing self-fertilisation in finite populations as a result of random mating among gametes. Fitness differences between the mating-system morphs were determined by inbreeding depression and pollen discounting in a context-dependent manner. Simulation results showed evidence of frequency dependence in the action of pollen discounting and inbreeding depression in finite populations. In particular, as a result of selfing in outcrossers through random mating among gametes, selfers experienced a "fixation bias" through drift, even when the mating-system locus was selectively neutral. In a metapopulation, high colony turnover generally favoured the fixation of the outcrossing morph, because inbreeding depression reduced opportunities for colony establishment by selfers through seed dispersal. Our results thus demonstrate that population size and metapopulation processes can lead to evolutionary dynamics involving pollen and seed dispersal that are not predicted for large populations with stable demography.     

Inbreeding depression and heterosis in a subdivided population: influence of the mating system

We investigate the joint effects of gene flow and selfing on the level of inbreeding depression, heterosis and genetic load in a subdivided population at equilibrium. Low gene flow reduces inbreeding depression and substantially increases heterosis. However, in highly self-fertilizing populations, inbreeding depression is independent of the amount of gene flow. When migration occurs via pollen, consanguinity of the reproductive system could have a negative influence on subpopulation persistence, in contrast to the case of isolated populations. However, with only seed migration, genetic load and heterosis depend mildly on the mating system. From an evolutionary point of view, we reach two main conclusions: first, outcrossing is selected for if gene flow is low; second, intermediate levels of gene flow could promote mixed mating systems, especially when migration occurs through pollen.     

The impact of extensive clonal growth on fine-scale mating patterns: a full paternity analysis of a lily-of-the-valley population (Convallaria majalis)

Background and Aims

The combination of clonality and a mating system promoting outcrossing is considered advantageous because outcrossing avoids the fitness costs of selfing within clones (geitonogamy) while clonality assures local persistence and increases floral display. The spatial spread of genetically identical plants (ramets) may, however, also decrease paternal diversity (the number of sires fertilizing a given dam) and fertility, particularly towards the centre of large clumped clones. This study aimed to quantify the impact of extensive clonal growth on fine-scale paternity patterns in a population of the allogamous Convallaria majalis.

Methods

A full analysis of paternity was performed by genotyping all flowering individuals and all viable seeds produced during a single season using AFLP. Mating patterns were examined and the spatial position of ramets was related to the extent of multiple paternity, fruiting success and seed production.

Key Results

The overall outcrossing rate was high (91 %) and pollen flow into the population was considerable (27 %). Despite extensive clonal growth, multiple paternity was relatively common (the fraction of siblings sharing the same father was 0·53 within ramets). The diversity of offspring collected from reproductive ramets surrounded by genetically identical inflorescences was as high as among offspring collected from ramets surrounded by distinct genets. There was no significant relationship between the similarity of the pollen load received by two ramets and the distance between them. Neither the distance of ramets with respect to distinct genets nor the distance to the genet centre significantly affected fruiting success or seed production.

Conclusions

Random mating and considerable pollen inflow most probably implied that pollen dispersal distances were sufficiently high to mitigate local mate scarcity despite extensive clonal spread. The data provide no evidence for the intrusion of clonal growth on fine-scale plant mating patterns.     

INDIVIDUAL VARIATION IN THE VIGOR OF SELF POLLEN AND SELFED PROGENY IN HIBISCUS MOSCHEUTOS (MALVACEAE)

In some self-compatible species, self pollen tubes grow more slowly than outcross pollen, presumably leading to low selfing rates when mixtures of self and outcross pollen reach the stigma simultaneously. Here we show that the competitive ability of self pollen differed among individuals of Hibiscus moscheutos. Self pollen tubes grew slower than outcross pollen in three plants, faster than outcross pollen in four plants, and showed no difference in five other plants (based on rates of callose plug formation). Levels of inbreeding depression were examined by comparing progeny from self and outcross pollinations in seven maternal families. Self pollination led to reduced seed number in only one maternal family, and a slight decrease in seed size was seen in two maternal families. Considerable inbreeding depression occurred later in the life cycle, and the degree of inbreeding depression varied among maternal families of 6-week-old plants. Our results demonstrate the potential for unpredictable effects of pollen competition on individual selfing rates, which in turn may affect progeny vigor. This complex situation contrasts with previous reports of species in which outcross pollen consistently outcompetes self pollen (cryptic self-incompatibility).     

Contemporary pollen flow,mating patterns and effective population size inferred from paternity analysis in a small fragmented population of the Neotropical tree <Emphasis Type="Italic">Copaifera langsdorffii</Emphasis> Desf. (Leguminosae-Caesalpinioideae)

Pollen flow and dispersal patterns were investigated in a small, isolated forest fragment of the Neotropical insect pollinated tree Copaifera langsdorffii, using paternity analysis and eight microsatellite loci. We also investigated the coancestry and effective population size of progeny arrays for conservation and environmental restoration purposes. Open-pollinated seeds were collected from 15 seed trees within the forest fragment, in which all adult trees were mapped, measured and genotyped. Twenty seeds were also collected from the nearest neighbor tree located 1.2 km from the forest fragment. Our results show that levels of genetic diversity were significantly higher in adults than offspring and significant levels of inbreeding were detected in offspring (F = 0.226). From paternity analysis, we observed low levels of selfing (s = 8%) and pollen immigration (m = 8%) in the fragment, but very high levels were detected for the isolated tree (s = 20%; m = 75%), indicating that the population and the tree are not reproductively isolated and are connected by patterns of long distance pollen dispersal (maximum detected 1,420 m). Within the forest fragment, the pattern of pollen dispersal was a near neighbor pattern with 49% of the pollen being dispersed within 50 m. The effective population size of the progeny array was low, indicating the need to collect seeds from a large number of seed trees (at least 76) for conservation purposes. The results show that the spatial isolation of the population and isolated tree due to forest fragmentation has not disrupted genetic connectivity; however, spatial isolation does seem to increase selfing and correlated mating.     

交配距离与父本数量对野慈姑有性繁殖的影响

植物的有性繁殖是生活史和进化的核心, 母本及环境对繁殖的影响固然重要, 父本的贡献亦不容忽视。父本来源与多样性对坐果结籽和后代质量的影响明显, 但由于不同物种或种群的繁殖特点和进化历程, 往往也会呈现其独特性。该研究旨在探究挺水植物野慈姑(Sagittaria trifolia)是否存在自交或远交衰退, 以及父本数量对其繁殖和后代表现的影响。通过人工控制授粉, 设置自交、近距离异交(<50 km)、远距离异交(＞200 km) 3种交配距离以及单、双两类父本数量, 共计5种授粉处理, 观测野慈姑坐果概率、单果种子数量、种子面积、萌发率(2018和2019年)、幼苗芽长(2018和2019年)共7项指标。结果显示: 不同距离的交配对野慈姑的坐果概率、单果种子数量、种子面积、萌发率、幼苗芽长没有显著影响, 野慈姑未出现明显的自交衰退或远交衰退现象。父本数量的增加对野慈姑的结实数量(坐果概率、单果种子数量、种子面积)无影响, 但结实质量方面, 双父本处理的种子萌发率略高于单父本处理。综上所述, 交配距离与父本数量对野慈姑的繁殖表现影响较小, 这可能与其频繁的自交历史、远距离的基因扩散以及高度相似的水生环境有关; 同时, 该研究基于20余个自然种群的基因型, 研究结果也表明了野慈姑物种水平有性繁殖的优异和稳定性。     

Mating patterns and contemporary gene flow by pollen in a large continuous and a small isolated population of the scattered forest tree Sorbus torminalis

The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower percentage of flowering trees, showed less intense flowering, lower fruiting, less developed seeds per fruit, increased selfing and received less immigrant pollen. However, the negative inbreeding coefficients (F(IS)) of offspring indicated that this did not result in inbred seed at the population level. We also show that flowering, fruiting and pollen flow patterns varied among years, the latter being affected by the size of individuals. Though our study was unreplicated at the factor level (i.e. isolated vs non-isolated populations), it shows that small and spatially isolated populations of S. torminalis may also be genetically isolated, but that their progeny is not necessarily more inbred.     

The effect of pollination mode on seed performance of Gentiana lutea: a laboratory evaluation of seed germinability

Gentiana lutea L. (yellow gentian, Gentianaceae) is a protected orophyte of central and southern Europe, mainly threatened by the uncontrolled collection of its rhizome, used in traditional medicine and for liquor production. The species is self‐compatible, but outcrossing mediated by pollinators is needed to obtain a viable progeny. In this study, we considered five natural populations belonging to the four subspecies of G. lutea. We performed controlled pollinations in the field (self‐ versus cross‐pollination) followed by seed germination tests in laboratory conditions, adding a solution of gibberellic acid, in order to evaluate the seed performance. A cumulative index of inbreeding depression was calculated considering maternal reproductive output as well as seed performance traits. Seed weight and seed germination performance was similar between seeds resulting from naturally pollinated and pollen‐augmented flowers and higher compared to selfed flowers, highlighting a disadvantage of selfing and the importance of cross‐pollen transfer in natural conditions. However, in the small and isolated population of G. lutea subsp. symphyandra on Mt Grande we found a general reduction in seed germination rate, likely due to increased selfing or mating among close relatives as a consequence of a severe bottleneck. We discuss our results with regards to implications for conservation practices.