Recent polyploidization and a trans-Nullarbor biogeographic boundary discovered in an ancient flowering plant

Relatively little is known about the evolutionary history of Hydatellaceae (Nymphaeales), despite considerable recent attention following the family’s taxonomic realignment away from the monocots. The family is now recognized as a major component of the ANITA or ANA grade (named after five of the six constituent families, or the three component orders, Amborellales, Nymphaeales and Austrobaileyales), which collectively defines the deepest divisions in flowering-plant phylogeny. The family comprises approximately 12 species of Trithuria (the sole genus) that are restricted to Australia, New Zealand and India. Despite the ancient origin of its stem lineage, the crown clade of the family diversified relatively recently (around the early Miocene), and much of its biogeographic distribution may be explained by recent long-distance dispersal events.

In this study, we examined patterns of genetic diversity across most of the range of Trithuria submersa, the most widespread member of Hydatellaceae using microsatellite (expressed-sequence tag simple-sequence repeat; EST-SSR) markers that we developed from the transcriptome assembly. Trithuria submersa is the most broadly distributed species in the family, as it occurs in southwestern (SW) and southeastern (SE) Australia (including Tasmania), two temperate sclerophyll biomes divided on the mainland by the Nullarbor plain, a ~750 km gap of arid limestone plateau.

Figure1_1110a) map of T. submersa sampling populations in Australia and the 750km arid zone separating them.The SW and SE populations are currently separated by the central arid zone and the Nullabor Plain. b) map of overall allelic richness across the geographic range.





Patterns of “allelic” variation (no more than two variants per EST-SSR genotype) and recently published chromosomal evidence are consistent with a polyploidization event and substantial homozygosity underlying fixed heterozygote SSR genotypes that in turn reflects a selfing mating system. Our finding of significant clustering of SW vs. SE Australian populations, together with the high levels of genetic differentiation between regions observed in the AMOVA analysis, provides further corroboration of the longstanding hypothesis that Australian SW and SE biomes were isolated long ago by the mid-Miocene aridification and by the uplift of the Nullarbor Plain as an edaphic barrier. This raises the question of whether Trithuria submersa in the two regions should be treated as different species, consistent with SE vs. SW lineages of this species present in both plastid and ITS gene trees.

We also applied transcriptome-based evidence to address whether it has experienced polyploidization in its recent ancestry, as suggested by recent karyotypic evidence. A transcriptome-based Ks plot revealed at least one recent polyploidization event, consistent with fixed heterozygous genotypes representing underlying sets of homeologous loci.


Our gene duplication analysis supports recent whole genome duplication in the evolutionary history of T. submersa.




Future directions

Our study provides the first insights into patterns of genetic diversity in a species of Trithuria and may be useful for mating-system studies, species-identification and other downstream studies in Hydatellaceae, an ancient ANA-grade lineage. We also made an initial assessment of cross-species transferability of the microsatellite markers in all congeners as a tool for future studies on the genetic structure in this family. The SSR markers developed show a moderate transferability to other species, which is highest for T. occidentalis and T. bibracteata, two species that are evolutionarily most closely related to T. submersa.

Read the full article on New Phytologist: by Isabel Marques, Sean A. Montgomery, Michael S. Barker, Terry D. Macfarlane, John G. Conran, Pilar Catalan, Loren H. Rieseberg, Paula J. Rudall, and Sean W. Graham (or click here for the pdf: New Phytologist Marques et al_trithuria).


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