Conservation implications of widespread polyploidy and apomixis: a case study in the genus Pomaderris (Rhamnaceae)

Abstract

Polyploidy resulting from whole genome duplication is common in plants and is increasingly being recognised as a critical factor in conservation actions, particularly when within-species variation in ploidy exists. Pomaderris (Rhamnaceae), a genus of 70 species endemic to Australia and New Zealand, has many species listed as threatened and that are subject to conservation management but of unknown ploidy. To provide a better understanding of polyploidy in the genus we sampled 36 of 70 species of Pomaderris and used flow cytometry to establish genome sizes and infer ploidy. Additionally, to examine within-species variation, we screened 104 individuals of eight rare species subject to conservation management. We did not find evidence for infraspecific variation in ploidy, suggesting that from a cytological perspective, conspecific individuals from the screened populations do not need to be kept geographically separated in conservation management. There is, however, considerable variation among species, with genome sizes suggesting the occurrence of diploidy, triploidy, tetraploidy and hexaploidy. Finding several species to be triploid but capable of seed production, we then explored reproductive biology using the flow cytometric seed screen. Results suggested that triploid species produce seeds asexually, as previously reported for two New Zealand congeners. While asexual reproduction through apomixis is generally a means of odd-ploid taxa overcoming sterility, we found that more than half of examined diploids and tetraploids also produced seeds asexually. Asexual reproduction means genetic diversity is potentially low, and these results should therefore be considered in future conservation actions and seed sampling designs.