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A reconsideration of relationships among Japanese Trillium species based on karyology and AFLP data

hybridization

Natural hybridization and subsequent polyploidization have been recognized as important factors in plant speciation. The genus Trillium contains about 45 species, of which 11 occur in eastern Asia, and the remainder in North America. All species in North America are known to be diploid, whereas the Asiatic species represent past hybridization and polyploidization events, and all but one species are polyploids. The chromosome analysis of the species was employed for its clarity, and the genealogical relationships of the Japanese Trillium has been estimated by caryotype analysis. However, the estimation remains indistinct, because it predicts non-existing species as ancestors, and no analysis has been conducted for a recently found species. Therefore, we conducted AFLP (Amplified Fragment Length Polymorphism) analysis to reconsider the relationships among the Japanese Trillium species. From the analysis, it was clarified that the relationships can be explained by hybridization and polyploidization among the existing Trillum species.

Kubota S, Kameyama Y, Ohara M (2006) A reconsideration of relationships among Japanese Trillium species based on karyology and AFLP data. Plant Systematics and Evolution 261:129-137. [PDF]


Mating system evolution in Trillum camschatcense

T. camschatcense

Although many plant species possess physiological self-incompatibility (SI); an obligate outcrossing system, numbers of species are known to have self-compatibility (SC), which enables self-fertilization. Genomic analyses in Brassicaceae and Solanaceae have pointed out that the evolutionary shift of the mating system results from the breakdown of the SI gene. Thus, it is generally believed that the evolution is one way, from SI to SC. However, because the reverse case has been reported in other families, such as Asteraceae, generality of this evolutionary trend remains unclear.
Trillium camschatcense (photo) is a hermaphroditc perennial, and populations of this species possess either SC or SI, according to their geographical location. Because the distinct mating systems distributes in geographically close area, T. camschatcense should be appropriate for studying the evolution of SI and SC.

Adaptive significance of self-fertilization

Theories predict that the evolutionary shift from SI to SC should occur because selfing is advantageous over outcrossing in terms of genetic transmission and assurance of seed production under pollen-limited circumstances. Here we used several SC and SI populations of T. camschatcense to investigate the adaptive significance and the evolutionary basis of self-fertilization. Pollination experiments and genetic analyses revealed that selfing does not provide fitness reproductive assurance; instead it disturbed the reproductive sucess from outcrossing.

Kubota S, Kameyama Y, Ohara M (2006) Characterization of six microsatellite markers in Trillium camschatcense using a dual-suppression-polymerase chain reaction technique. Molecular Ecology Notes 6:1135-1137. [PDF]

Kubota S, Kameyama Y, Hirao AS, Ohara M (2008) Adaptive significance of self-fertilization in a hermaphroditic perennial, Trillium camschatcense (Melanthiaceae). American Journal of Botany 95:482-489. [PDF]

Evolution of self-incompatibility from self-compatibility

From our ecological surveys, selfing in the SC populations of T. camschatcense did not possess any fitness advantage. Instead, selective pressure which favors an obligate outcrossing system existed in these populations. Therefore, the widely accepted evolutionary direction (from SI to SC) was reconsidered in this species in terms of molecular phylogeny. Although phylogenetic analyses alone did not provide convincing evidence, combined results from ecological, geographical, and phylogenetic analyses pointed out that the SI populations were monophyletic, deriving from the SC populations.

Kubota S, Ohara M (2009) The evolution of self-incompatibility from self-compatibility in a hermaphroditic perennial, Trillium camschatcense (Melanthiaceae). Journal of Plant Research 122:497-507. [PDF]

Physiological mechanism of self-incompatibility

SC and SI

SI in the Trillium genus has also been reported in North American species. The North American species possess an incomplete (or "leaky") SI, which results in occasional selfing. In contrast, T. camschatcense in Japan is either fully self-compatible (photo-left: SC population), or completely self-rejecting (photo-right: SI population). Using histological staining and SEM (scanning electron microscope) observations, I plan to clarify the physiological mechanism of SI in T. camschatcense. Comparison with the North American Trillum species should provide insights into clarifying the evolution of SI in this genus. In addition, the results would help characterizing the SI gene in T. camschatcense.[in progress]

Discovery of female individuals

male sterile

Although T. camschatcense has been considered as hermaphroditic, we have noticed individuals with degenerated anthers in some SC populations (photo). Because our previous work indicated that the SC populations were suitable for outcrossing, females could be adaptive in these populations. Here, we investigated whether the anther degenerated (AD) plants could be regarded as functional females, and if so, how their loss of male function is compensated. Morphological and genetic analyses demonstrated that AD plants were completely male sterile, but female fertile. Moreover, females achieved higher reproductive advantage than the hermaphrodites by avoiding severe inbreeding depression. In fact, females were rarely found in the SI populations, where hermaphrodites were free from inbreeding.

Kubota S, Ohara M (2009) Discovery of male sterile plants and their contrasting occurrence between self-compatible and self-incompatible populations of the hermaphroditic perennial Trillium camschatcense. Plant Species Biology 24:169-178. [PDF]

The impact of female frequency on population-level breeding system

female frequency

Selfing generally reduces genetic diversity, and estimated genetic diversity of T. camschatcense is often lower in the SC populations compared to the SI populations. However, because the newly discovered females showed vigorous seed production, frequency of the females may strongly affect the breeding system of the population. As expected, SC populations with abundant females possessed higher levels of genetic diversity. Reproductive contribution of the hermaphrodites in these SC populations were limited to pollen donation, thus forming a kind of sexual dimorphism.


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