Synchronic historical patterns of species diversification in seasonal aplocheiloid killifishes of the semi-arid Brazilian Caatinga

The Caatinga is the largest nucleus of seasonally dry tropical forests in South America, but little is known about the evolutionary history and biogeography of endemic organisms. Evolutionary diversification and distribution of terrestrial vertebrates endemic to the Caatinga have been explained by palaeogeographical Neogene episodes, mostly related to changes in the course of the São Francisco River, the largest river in the region. Our objective is to estimate the timing of divergence of two endemic groups of short-lived seasonal killifishes inhabiting all ecoregions of the Caatinga, testing the occurrence of synchronic events of spatial diversification in light of available data on regional palaeogeography. We performed independent time-calibrated phylogenetic molecular analyses for two clades of sympatric and geographically widespread seasonal killifishes endemic to the Caatinga, the Hypsolebias antenori group and the Cynolebias alpha-clade. Our results consistently indicate that species diversification took place synchronically in both groups, as well as it is contemporary to diversification of other organisms adapted to life in the semi-arid Caatinga, including lizards and small mammals. Both groups originated during the Miocene, but species diversification started between the Late Miocene and Early Pliocene, when global cooling probably favoured the expansion of semi-arid areas. Synchronic diversification patterns found are chronologically related to Tertiary palaeogeographical reorganizations associated to continental drift and to Quaternary climatic changes, corroborating the recent proposal that South American biodiversity has been continuously shaped between the Late Paleogene and Pleistocene.

卡廷加（Caatinga）是南美洲规模最大的季节性干旱热带森林核心区域，但学界对其特有生物的演化历史与生物地理学认知仍较为有限。此前针对卡廷加特有陆生脊椎动物的演化分化与分布格局的解释，多归因于新近纪古地理事件，尤以该区域最大河流——圣弗朗西斯科河（São Francisco River）河道变迁相关的影响最为突出。本研究旨在估算栖息于卡廷加所有生态区的两类特有一年生季节性鳉鱼的分化时间，并结合区域古地理学现有数据，检验其空间分化事件是否存在同步性。我们针对两类同域分布、地理分布广泛的卡廷加特有季节性鳉鱼支系——安氏小鰕鳉群（Hypsolebias antenori group）与辛诺勒毕鳉α支系（Cynolebias alpha-clade），分别开展了时间校准的系统发育分子分析。研究结果一致表明，两类支系的物种分化均具有同步性，且与适应半干旱卡廷加生境的其他生物（包括蜥蜴与小型哺乳类）的分化时间重合。两类支系均起源于中新世，但物种分化始于中新世晚期至上新世早期，彼时全球气候变冷或推动了半干旱区域的扩张。本次研究发现的同步分化模式，在时间序列上与大陆漂移相关的第三纪古地理重组以及第四纪气候变化存在关联，印证了近期提出的"南美生物多样性在古近纪晚期至更新世期间持续塑造"这一学术假说。