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Journal of Biogeography 36: 703-719 (2009)

The collision of the Indian plate with Asia: molecular evidence for its impact on the phylogeny of freshwater crabs (Brachyura: Potamidae)

H.-T. Shih, D. C. J. Yeo & P. K. L. Ng

Aim We used molecular data to answer the following questions: (1) Is morphology-based (and to some extent, geography-based) classification of the freshwater crab family Potamidae congruent with a molecular phylogeny? (2) What historical biogeographical event could have shaped this phylogeny?

Location Material from the entire geographical range of the family Potamidae was analysed, including specimens from East Asia (China, Taiwan, the Ryukyus), Southeast Asia, South Asia (northern India, the Middle East and Near East), North Africa, and southern Europe.

Methods Mitochondrial DNA sequences encoding 503 base pairs (excluding the variable regions) of the large subunit rRNA (16S rRNA) gene were obtained from 72 species belonging to 49 potamid genera, representing 51% of all known genera in this species-rich family. Sequences were compared by means of phylogenetic analyses (minimum evolution, Bayesian inference, maximum likelihood and maximum parsimony) and Bayesian relaxed molecular clock estimates.

Results The family Potamidae was found to be monophyletic with two major lineages, and there was support for the recognition of two mostly allopatric subfamilies, Potaminae and Potamiscinae. This is largely consistent with the current classification proposed. The 'Potamiscinae' clade comprised three subclades: (1) a well-supported 'eastern Asia' subclade that included species from the eastern part of the range (China, Taiwan, the Ryukyus, the Philippines, Indochina, Malay Peninsula, northern India and Myanmar/Burma); (2) a weakly supported 'Sunda Shelf islands' subclade that included species from the larger Southeast Asian islands on the Sunda Shelf (Borneo, Sumatra and Java); and (3) a 'Socotra' subclade that comprised only Socotrapotamon from Socotra Island, off the north-east coast of Africa.

Main conclusions The discrete distribution of the two subfamilies in Europe/Asia is hypothesized to be the result of vicariance due to the collision of the Indian tectonic plate with the Asian continent, and the orogeny that caused the separation of the two freshwater crab lineages around 22.8 Ma. Within the Potamiscinae, the 'Sunda Shelf islands' subclade separated from other potamiscines around 21.1 Ma; and the endemic fauna of the East Asian islands (Taiwan, the Ryukyus and mainland Japan) was isolated from the Asian continent c. 8.4 Ma, following the opening of the Okinawa Trough. The 'Socotra' subclade diverged from the 'eastern Asia' subclade at 19.1 Ma during the Miocene. Its taxonomic position, however, remains unclear as the members of this clade possess the key potamine character of a transverse ridge on thoracic sternite 8, suggesting that this may in fact be a relict potamid group.

Keywords Biogeography, East Asian islands, freshwater crabs, mtDNA, Okinawa Trough, plate tectonics, Potaminae, Potamiscinae, Sunda Shelf islands, vicariance.

Figure 1 Distribution of the family Potamidae, with the ranges of three subclades of the subfamily Potamiscinae: 'eastern Asia' (pale dotted line), 'Sunda Shelf islands' (pale solid line) (both iconized without 'ridge') and 'Socotra' subclades (iconized with 'ridge'), and the subfamily Potaminae (pale dashed line) (iconized with 'ridge'). The grey dots represent the collection sites, with the numbers representing the collection sites in Table 1. The thick dark dotted line is the supposed division between Potaminae and Potamiscinae; the dark solid line is Wallace's line. The icons used show the thoracic sternites; the transverse ridge being represented by the double horizontal lines.

Figure 2 Saturation plots of the number of transitions (s) and transversions (v) vs. divergence as measured by K80 for 16S rRNA of the family Potamidae.

Figure 3 A minimum evolution (ME) tree for the family Potamidae from Asia, Europe and Africa, and outgroups, based on 503 bp of 16S rRNA. Probability values at the nodes represent confidence values for ME, Bayesian inference (BI), maximum likelihood (ML) and maximum parsimony (MP). Only values > 50% for ME and BI methods, and > 40% for ML and MP methods are shown ('-' means not supported). The two subfamilies are shown for the species included (black ground at the right), with the 'eastern Asia', 'Socotra' and 'Sunda Shelf islands' subclades (grey ground near the right). The groups numbered 1-8 belong to the 'eastern Asia' subclade, and groups 'Borneo I', 'Borneo II' and 'SW Sunda' belong to the 'Sunda Shelf islands' subclade. The support values for each clade, subclade and group are shown in bold.

Figure 4 A chronogram for the family Potamidae based on 503 bp of 16S rRNA, with the divergence times estimated (shown at the main nodes) from the Bayesian relaxed molecular clock method, multidivtime (Thorne & Kishino, 2002). Calibration point 1 was set based on the time of the opening of the Okinawa Trough and calibration point 2 was based on the earliest possible occurrence of potamids. See text for detailed explanation. The  subclades and groups shown at the right are the same as in Fig. 3. Numbers above the nodes are million years ago (Ma).

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