Ancient mitogenomes reveal evidence for the late Miocene dispersal of mergansers to the Southern Hemisphere

Abstract

Mergansers are riverine and coastal piscivorous ducks that are widespread throughout North America and Eurasia but uncommon in the Southern Hemisphere. One species occurs in South America and at least two extinct species from New Zealand. It has been proposed that Southern Hemisphere mergansers were founded by independent dispersal events from the Northern Hemisphere. However, some morphological and behavioural evidence suggests that Southern Hemisphere mergansers may be closely related to one another. They share several characteristics that differ from Northern Hemisphere mergansers (e.g., non-migratory vs. migratory, sexual monochromatism vs. sexual dichromatism, long vs. short pair bonds). We sequenced complete mitogenomes from the Brazilian merganser and an extinct merganser from New Zealand--the Auckland Island merganser. Our results show the Brazilian and Auckland Island mergansers form a monophyletic clade with the common merganser, and that a sister relationship between Southern Hemisphere mergansers cannot be rejected. We cannot exclude the possibility that the Brazilian and Auckland Island mergansers descend from a single dispersal event from the Northern Hemisphere at least seven million years ago. Nuclear (palaeo)genomic data may help to resolve the relationship between living and extinct mergansers, including merganser fossils from New Zealand that have not been subjected to palaeogenetic analysis.