Abstract
Author SummaryEver since the first Europeans reached the Australian shores and were fascinated by the curious marsupials they found, the evolutionary relationships between the living Australian and South American marsupial orders have been intensively investigated. However, neither the morphological nor the more recent molecular methods produced an evolutionary consensus. Most problematic of the seven marsupial groups is the South American species Dromiciops gliroides, the only survivor of the order Microbiotheria. Several studies suggest that Dromiciops, although living in South America, is more closely related to Australian than to South American marsupials. This relationship would have required a complex migration scenario whereby several groups of ancestral South American marsupials migrated across Antarctica to Australia. We screened the genomes of the South American opossum and the Australian tammar wallaby for retroposons, unambiguous phylogenetic markers that occupy more than half of the marsupial genome. From analyses of nearly 217,000 retroposon-containing loci, we identified 53 retroposons that resolve most branches of the marsupial evolutionary tree. Dromiciops is clearly only distantly related to Australian marsupials, supporting a single Gondwanan migration of marsupials from South America to Australia. The new phylogeny offers a novel perspective in understanding the morphological and molecular transitions between the South American and Australian marsupials.
Highlights
The phylogenetic relationships among the four Australasian and three South American marsupial orders have been intensively debated ever since the small species Dromiciops was taxonomically moved from Didelphimorphia into the new order Microbiotheria and the cohort Australidelphia was erected based on ankle joint morphology [1]
Sequence-based attempts to resolve the positions of the South American opossums (Didelphimorphia) and the shrew opossums (Paucituberculata), which appear some few million years apart in the South American fossil layers close after the Cretaceous-Tertiary boundary [2], relative to Australidelphia have so far been futile (e.g., [3,4])
Several studies suggest that Dromiciops, living in South America, is more closely related to Australian than to South American marsupials
Summary
The phylogenetic relationships among the four Australasian and three South American marsupial orders have been intensively debated ever since the small species Dromiciops was taxonomically moved from Didelphimorphia into the new order Microbiotheria and the cohort Australidelphia was erected based on ankle joint morphology [1]. Australidelphia comprises the four Australasian marsupial orders and the South American order Microbiotheria, a close relationship suggesting a complex ancient biogeographic history of marsupials. Sequence-based attempts to resolve the positions of the South American opossums (Didelphimorphia) and the shrew opossums (Paucituberculata), which appear some few million years apart in the South American fossil layers close after the Cretaceous-Tertiary boundary [2], relative to Australidelphia have so far been futile (e.g., [3,4]). Because young retroposed elements can insert into older elements, but older, inactive elements are not capable of inserting into younger ones, nested retroposon insertion patterns
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