Background: Sri Lanka is a continental island separated from India by the Palk Strait, a shallow-shelf sea, which was emergent during periods of lowered sea level. Its biodiversity is concentrated in its perhumid south-western ‘wet zone’. The island’s freshwater fshes are dominated by the Cyprinidae, characterized by small diversifcations of species derived from dispersals from India. These include fve diminutive, endemic species of Pethia (P. bandula, P. cumingii, P. melanomaculata, P. nigrofasciata, P. reval), whose evolutionary history remains poorly understood.
Here, based on comprehensive geographic sampling, we explore the phylogeny, phylogeography and morphological diversity of the genus in Sri Lanka.
Fig. 1 a Sri Lanka, showing the geographical origin of samples of Pethia nigrofasciata and P. bandula used in this study. The thin black lines indicate basin boundaries, while the bold black line indicates the 2000-mm isohyet, which encompasses the wet zone. Numbers on the map (a) represent the sampling localities listed in Table 1. b close-up of the sampling localities for P. nigrofasciata in the headwaters of the Attanagalu basin neighboring the type locality of P. bandula. These populations show a mix of meristic characters and color patterns intermediate between P. nigrofasciata and P. bandula. In d–r, specimens with complete and incomplete pored lateral-line scales are represented by black and white squares, respectively while those with complete bar beneath dorsal fn and those without or an incomplete are represented by black and white circles, respectively. Numbers on the map (b) are referenced to the specimens illustrated in (g–n) in parentheses. c Live coloration of P. bandula. d Holotype of P. bandula, ZRC 38,483, 34.6 mm SL, e paratype CMK 7146C, 31.6 mm SL, and f paratype, CMK 7146D, 30.8 mm SL of P. bandula. Pethia cf. nigrofasciata from the Attanagalu basin in g–n. g DZ5353F, 40.3 mm SL (2). h DZ4452C, 38.8 mm SL (5). i DZ4452A, 36.9 mm SL (5). j DZ5352B, 39.0 mm SL (2). k DZ5350E, 42.6 mm SL (4). l DZ5351I, 37.1 mm SL (3). m DZ5350G, 35.7 mm SL (4). n DZ5351A, 34.8 mm SL (3). o Live coloration of P. nigrofasciata. Pet
Results
The phylogenetic analyses, based on mitochondrial and nuclear loci, recover Sri Lankan Pethia as polyphyletic. The reciprocal monophyly of P. bandula and P. nigrofasciata, and P. cumingii and P. reval, is not supported. Pethia nigrofasciata, P. cumingii, and P. reval show strong phylogeographic structure in the wet zone, compared with P. melanomaculata, which ranges across the dry and intermediate zones. Translocated populations of P. nigrofasciata and P. reval in the Central Hills likely originate from multiple sources. Morphological analyses reveal populations of P. nigrofasciata proximal to P. bandula, a narrow-range endemic, to have a mix of characters between the two species. Similarly, populations of P. cumingii in the Kalu basin possess orange fns, a state between the red-fnned P. reval from Kelani to Deduru and yellow-fnned P. cumingii from Bentara to Gin basins.
Conclusions
Polyphyly in Sri Lankan Pethia suggests two or three colonizations from mainland India. Strong phylogeographic structure in P. nigrofasciata, P. cumingii and P. reval, compared with P. melanomaculata, supports a model wherein the topographically complex wet zone harbors greater genetic diversity than the topographically uniform dry-zone. Mixed morphological characters between P. bandula and P. nigrofasciata, and P. cumingii and P. reval, and their unresolved phylogenies, may suggest recent speciation scenarios with incomplete lineage sorting, or hybridization.
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