What ecological factors favor the shift from distyly to homostyly? A study from the perspective of reproductive assurance

Aims Distyly is one of the most widespread floral polymorphisms pro-moting cross-fertilization.Evolutionary transition from obligate cross-fertilized distyly to predominantly self-fertilized homostyly is frequently documented in various groups.However,empirical stud-ies concerning the ecological factors connected with this transition are still lacking.Primula chungensis,suggested to be evolving from distyly to homostyly,provides an ideal model for the study of the ecological factors concerned with this transition.We study P.chun-gensis to understand if autonomous self-fertilization would provide reproductive assurance for the self-fertilized homo-styled morph in the field.Methods the incompatibility features of P.chungensis were tested with hand-pollination experiments.We compared the capacity of autonomous self-fertilization between the distylous and homo-styled morph of P.chungensis in the field by excluding the pollinators with bags.In addition,the degrees of herkogamy of some P.chungensis plants were between the short-styled and homo-styled morphs.these plants were studied to understand whether they were able to obtain greater reproductive assurance when the herkogamy in the flowers was reduced.Important Findings All three morphs of P.chungensis were highly self-and intra-morph compatible.the degree of herkogamy positively correlated with the capacity for autonomous self-fertilization.A negative correlation between the degree of herkogamy and the magnitude of pollen limitation was found,but no significant correlation was found between the degree of herkogamy and the contribution of cross-fertilization to overall fertilization.this study suggests that reducing the degree of herkogamy can significantly increase the reproductive assurance for a self-compatible plant.Our results provided evidence that the homo-styled morph of P.chungensis had the highest capacity for autonomous self-fertilization and the highest seed production in the field,because autonomous self-fertilization provided reproduct-ive assurance for the homo-styled morph.this may cause selection towards the transition from distyly to homostyly.

Pollinator-mediated self-pollination and reproductive assurance in an isolated tree of Magnolia grandiflora L.

Background:The ability to produce seeds when pollinators or potential mates are scarce is one of the principal advantages of self-pollination in flowering plants.However,the role of pollinators mediating self-pollination to ensure seed set when pollen or potential mates are limited has received less attention.This study examined the reproductive consequences and involvement of pollinators in assuring seed set in an isolated tree of Magnolia grandiflora,a predominantly outcrossing species.Methods:We tested various aspects of reproduction such as flower density,floral rewards,stigma receptivity,pollinator abundance and behaviour,fruit set and seed set,in two successive reproductive years(2015-2016).Results:Flowers of M.grandiflora possess a suite of traits facilitating pollinator-mediated self-pollination(PMS),although the chances of autonomous self-pollination are reduced due to herkogamy,spatial separation of anther and stigma.The mean pollen production/flower was 5,152,289±285,094 with a pollen-to-ovule ratio of 39430±164.We found a significant positive correlation between number of visits and seed production for bees(r=0.5099,p=0.0007)and beetles(r=0.7159,p=0.00001),indicating these are effective at PMS.There was a significant negative correlation for thrips(r=-0.3206,p=0.044)and no correlation for flies or spiders.The percent fruit set was 100%and the seed set per ovule ranged between 19 and 20%.Conclusions:PMS will guarantee reproductive assurance and mitigate the effect of reduced mates or pollen limitation which is expected to increase the fecundity and establishment of individual trees in geographically isolated locations.

Potential Autonomous Selfing in Gesneria citrina (Gesneriaceae),a Specialized Hummingbird Pollinated Species with Variable Expression of Herkogamy

Species with mixed mating systems often demonstrate variable expression of breeding system characteristics and thus represent the opportunity to understand the factors and mechanisms that promote both outcrossed and selfed seed production. Here, we investigate variation in levels of herkogamy （variation in stigma-anther separation distance） in a Puerto Rican population of hummingbird pollinated Gesneria citrina Urban. There is significant variation in herkogamy levels among individuals of this species and stigma-anther separation is negatively associated with the ability to set fruits and seeds in the absence of pollinators. The variation in levels of herkogamy may represent a mechanism to ensure the production of some self-fertilized progeny in the absence of hummingbird pollinators. We also describe a novel breeding system in G. citrina, where stamens elongate over time to reach stigma height, but stamen elongation is accelerated by pollination. These results suggest that once the flowers are pollinated, stamen elongation may favor increased pollen removal and siring success, while the reduction in stigma-anther distance no longer imposes the risk of interference between male and female functions. We discuss our findings of breeding system variation in the context of pollination system evolution in an island setting （Antillean islands）.