Recent species in old Islands: the origin of introduced populations of Litoria aurea (Anura: Hylidae) in New Caledonia and Wallis

New Caledonia is a megadiverse tropical island in the southwest Pacific, however, inhabited by only one species of amphibian, Litoria aurea (Hylidae). We used both molecular (CO1 and ND4 gene sequencing) and morphometric data to explore its geographical origin and timing of colonisation. We tested whether this species arrived through transoceanic dispersal before human arrival in the island, or recently through anthropogenic introduction. We found a weak phylogeographical structure within this species, and lower haplotype diversity in New Zealand, New Caledonia and Wallis compared to Australia. No significant genetic differentiation was found between pairs of populations in New Caledonia and Wallis, or between pairs of population from these two islands. We observed a high level of morphometric differentiation between Australian and island populations, and a low level of morphometric differentiation between island populations. Our results support an Australian origin for insular frogs. The possibility of a trans-marine dispersal from Australia to New Caledonia and/or Wallis in-between the Eocene and the Pleistocene cannot be favoured, given the low level of genetic differentiation. Our results are consistent with a recent human introduction, most likely during European times. Our data support the historical absence of amphibians in the old island New Caledonia, and is consistent with the new biogeographical paradigm that this island was totally re-colonized after emergence in Eocene. More studies are necessary to explain the success of this frog in oceanic islands, where it is widespread and abundant, compared to Australia, where it is declining.

新喀里多尼亚（New Caledonia）是西南太平洋的超多样热带岛屿，却仅栖息有一种两栖动物——绿树蛙（Litoria aurea，隶属雨蛙科Hylidae）。本研究结合分子生物学（CO1与ND4基因测序）与形态测量学数据，探究该物种的地理起源与殖民该岛的时间节点。我们验证了两种假说：该物种是否在人类抵达该岛前通过跨洋扩散抵达，抑或是近期由人类活动引入。研究发现该物种种内系统地理结构较弱，且新西兰、新喀里多尼亚与瓦利斯群岛的单倍型多样性均低于澳大利亚种群。新喀里多尼亚与瓦利斯群岛的种群间，以及这两座岛屿的种群两两之间，均未检测到显著的遗传分化。我们还观察到，澳大利亚种群与各岛屿种群间存在高度的形态测量学分化，而各岛屿种群间的形态测量学分化程度则较低。本研究结果支持该岛屿蛙类的澳大利亚起源假说。鉴于遗传分化程度较低，该物种在始新世至更新世期间从澳大利亚跨海扩散至新喀里多尼亚和/或瓦利斯群岛的可能性无法成立。研究结果与“近期由人类引入”的假说相符，且引入时间极可能处于欧洲殖民时期。本研究数据支持“历史上新喀里多尼亚这座古老岛屿原本并无两栖动物”的结论，且与新生物地理学范式相符——该岛屿在始新世形成后曾完全经历一次重新殖民过程。相较于种群数量下降的澳大利亚本土种群，该蛙在大洋岛屿中分布广泛且数量繁盛，其成功定殖的机制仍需更多研究加以阐释。