Biogeography of Australian chenopods: landscape in the evolution of an arid flora

Abstract

Chenopod taxa are globally prominent in deserts and on coastlines which suggests an evolutionary link, known as a littoral connection, and that remains unexplained. Australia is a global centre of chenopod diversity, where serial invasions of eleven immigrant clades over the last 16 million years, produced ca. 300 species within the continent. Six of these clades adaptively radiated in the Late Miocene-Pliocene within the continental interior. Assuming a littoral connection as a working premise, Australian landscape history can inform the course of chenopod evolution. Almost all species are endemic and each clade must exhibit the three elements of the connection. Coasts and deserts share related taxa, marshy, sandy or saline habitats and adaptive traits for habitat occupation. Phytogeographic analysis defines three groups of Australian chenopods. A Subcontinental Arid-Mediterranean Group has 97% of all species with Centres of Diversity in the west (Yilgarn) and east (Eyre-Murray). Sharing 129 species, the centres are strongly linked through the Great Victoria Desert, suggesting a common migration route. The chenopod poor Northern Tropical and Eastern Highlands groups suggest the barriers to range expansions. Diversification largely occurred inland, with 246 species being remote from coastlines. Only the Scleroblitum Clade lacks coastal taxa, but it has an estuarine ancestor. Under the habitat element of a littoral connection, migration landscapes should be marshy, sandy or saline. Chenopod taxa mainly inhabit Riverine Desert (141 species) and Desert Lake (113 species) within continental drainage systems. Most coastal species are of diverse coastal habitats, with a capacity for inland range expansion through marshy ecosystems. Riverine Desert developed as chenopod clades arrived, connecting coastlines to both Centres of Diversity. Youth and discontinuity of Desert Lake precludes initial continental migration but promoted Pleistocene speciation. Sand Desert has relatively few species; a Pleistocene age postdates inland range expansion and its formation contributed to species population disjunction in rich clades. Molecular phylogeny of Australian Atriplex Clade 1 reveals west to east migrations in two lineages after immigrant landfall on the west coast. One lineage migrated through inland Australia, initially diversifying in Stony Desert. The other lineage is limited to the southern coast and Western Australian catchments. Poor phylogenetic resolution in Atriplex Clade 2 precluded further interpretation, but initial diversification post-dates that of Atriplex Clade 1. Coastal species possibly have basal phylogenetic positions in both clades. These findings suggest evolutionary stages of Australian chenopods involve: coastal landfall, initial inland migration through declining palaeodrainage systems and diversification in drying landscapes, along two possible biogeographic patterns. Either an initial, widespread inland migration fragmented into the Yilgarn and Eyre-Murray Centres, or separate, incipient Centres subsequently enlarged and exchanged species. Species extinction may be associated with dune field formation and loss of ancestral coastal taxa. Riverine Desert provided the species pool for significant speciation in Desert Lake and Stony Desert land types. Each immigrant clade observes the taxon, habitat and trait elements of a littoral connection, which remains the most likely evolutionary scenario for these iconic Australian arid taxa.