Revisiting the bicontinental allopolyploid origin of Australasian Lepidium species with amplicon sequencing of 15 nuclear genes

Abstract

New Zealand is home to 16 endemic species of Lepidium (Brassicaceae), including two species considered to have become extinct since European colonisation and an additional species shared with Australia. Previous meiotic chromosome counts suggest at least some are high polyploids. A hybrid origin has been proposed for several of them, with African and American species implicated as parents. We generated amplicon sequence data for 15 genes derived from targeted enrichment data from a selection of species endemic to New Zealand, and of African, American, and Australian origin naturalised in New Zealand. These were analysed, along with publicly available genomic data from other Lepidium species. Our results are consistent with earlier work in supporting two main groups of endemic New Zealand Lepidium, corresponding to different dominant nrDNA sequences. Our data suggest that these two groups differ in ploidy (one octoploid, the ploidy of the other uncertain). The octoploid species combine four distinguishable subgenomes and the other species may be the products of allopolyploidisation between octoploid species and a diploid species closely related to, if not, L. africanum. Previously proposed links to American Lepidium are also supported, with at least two Australasian Lepidium subgenomes related to subgenomes present in American polyploid species. The affinities of two further subgenomes present in Australasian Lepidium remain to be determined, but they may also be of American origin. One New Zealand endemic species, L. naufragorum, displays sequences broadly consistent with it being the octodecaploid product of hybridisation between octoploid and decaploid New Zealand groups.