The Phylogeny And Diversity Of The Genus Crab

Diversification as the balance between speciation and extinction is the formation and dynamic process of species diversity. Diversification rate is the difference value between speciation and extinction rates. Diversification and its rate heterogeneity provide an important route for us to understand the origin and dynamic development of extant species diversity. True freshwater crabs play a key role in maintaining inland freshwater ecosystem as freshwater benthic macro-invertebrates. True freshwater crabs have amphibious life, development directly and have relatively poor dispersal ability. Their distribution is sensitive to the topography, climate fluctuations and other factors, which making them the ideal model for researching phylogenetic biogeography, diversification and its driving mechanism. Sinopotamon Bott, 1967 is the most speciose freshwater crab genus globally and widely distributed true freshwater crab genus in East Asia. Its striking species diversity and large geographic range are unique and constitute a compelling and unorthodox system on which to test diversification in groups supported by freshwater habitats in China and even in East Asia. However, there is still a lack of molecular phylogeographic and diversificationstudies on the whole genus. Our extensive sampling includes about 76% of the known Sinopotamon taxa (65 species), and nearly covers its entire distribution area (about 140 sampling sites). Based on mitochondrial COI and 16S rRNA, and nuclear 28S rRNA and H3 gene markers, we reconstructed the Sinopotamon phylogeny firstly.The ancestral geographic distribution and character state were reconstructed and the divergence time was estimated, which provide both a temporal and spatial framework for this genus. Multiple statistical approaches and mathematical models were used to conduct diversification analyses. The origin, dynamic development and driving mechanisms were discovered and discussed. The main conclusions were as follows:(1) Three main clades (Clades ?, ? and ?) that roughly correspond to their main geographic distribution ranges were recognized, in which Clade I was the sister group to a monophyletic group including Clades ? and ?. Clade ? was composed of eight species which are mainly distributed in the Sichuan Basin and surrounding mountains(SBSM) and all belong to the rounded lobes group.(2) The molecular phylogeny challenges the current view of the four major species groups based on the morphological differences in the terminal lobes of male first gonopod (G1). The phylogeny shows deep molecular genetic divergences within three of the four conventional morphologically defined groups, especially in the acute lobes group and the rounded lobes group. The ancestral character states reconstruction suggests that the evolution of rounded and acute lobes happened many times.(3) Species-level non-monophyly was found in the phylogenetic tree. The incidences in each of the three main clades were 12.5% (Clade ?), 22% (Clade ?) and 15% (Clade ?) respectively.(4) The most recent common ancestor (MRCA) of Sinopotamon most likely originated from the SBSM and subsequently dispersed throughout central and eastern China. The main rivers such as Yangtze River and its multiple tributaries may provide advantageous routes for dispersal of Sinopotamon crabs out of the Sichuan Basin and for their subsequent central and eastern China colonization. The MRCA of Clade ?most likely distributed in the lower reaches of the Yangtze River Basin and southeastern area, and its time could be inferred in the Pliocene, thus the Sinopotamon extending to the lower reaches of the Yangtze River Basin should not be later than this period.(5) Diversification rate heterogeneity through time, through space, and among clades was recognized. The lineage through time plot, rate through time plot,Birth-death likelihood analyses and rate shift configurations all suggest this genus especially Clade ? which mainly distributes in the middle and western area has undergone recent rapid diversification in Pleistocene. The high incidence of species-level non-monophyly found in Clade ? can be explained by its recent rapid radiation. Incomplete lineage sorting, introgressive hybridization and/or cryptic species caused by recent rapid diversification may be responsible for the species-level non-monophyly. Rapid diversification may also cause the heterogeneity of species diversity through space and among clades.(6) The climate fluctuations such as Pleistocene glacial-interglacial cycle,mid-Pleistocene transition and East Asia monsoon oscillations may be the main factors driving the burst of diversification. The continuous fluctuation between long-term warm humid and cold arid climates in Pleistocene may have facilitated a burst in species diversification. The heterogeneous rates among clades and through space may be due to the possible nonhomogeneous influence of climate oscillations on different geographic regions. The G1 lobe-morph has evolved repeatedly via multiple independent diversification events in Clade ? and Clade ?, especially in Clade ?. Additionally, numerous tributaries of the Yangtze River and other main river basins such as the Yellow River, and the highly heterogeneous topography of mountains may promote allopatric speciation. All of these are also likely contributing factors in Sinopotamon diversification. Morphological innovations, range expansion and geographical heterogeneity may all contribute to the diversification in Sinopotamon.(7) The results suggest that rapid diversification has played an important role in the Sinopotamon diversification processes. This study contributes to our knowledge on diversification of freshwater benthic macro-invertebrates in Chinese inland ecosystem.