In the rivers: Multiple adaptive radiations of cyprinid fishes (Labeobarbus) in Ethiopian Highlands

Abstract

Multiple repeated patterns of adaptive radiation were revealed in cyprinid fish inhabiting the compact geographic region of the ethiopian Highlands. We found four independently evolved radiations in the evolutionary hexaploid (2n = 150) Labeobarbus lineage based on matrilineal relationships of >800 individuals. Each radiation displayed similar patterns of mouth phenotype diversification, and included ecomorphs/species of the generalized, lipped, scraping (one or two), and large-mouthed (one to three) types. All radiations were detected in geographically isolated rivers, and originated from different ancestral populations. This is the first documented case in which numerous parallel radiations of fishes occurred-via different ways-in a riverine environment. Some radiations are very recent and monophyletic, while others are older and include ecomorphs that originated in separate mini flocks and later combined into one. The diversification bursts among Ethiopian Labeobarbus were detected in the mid-upper reaches of rivers (1050-1550 m above sea level), which likely offer ecological opportunities that include diverse habitats yet poor fauna (i.e. lower competition and relaxed selection). this promising example of parallel evolution of adaptive radiation warrants further investigation.In the pioneering work on South African cichlids and catfishes, Joyce et al. 21 and Day et al. 19 suggested that the riverine flocks are remnants of radiations that emerged in a paleolake. This suggestion is corroborated by the finding of the Lake Tanganyika endemics beyond the lake basin 33 . While some riverine radiations clearly have lacustrine origins, it is likely that other riverine radiations originate from sources other than lakes. Hence, we aimed to test whether the same environmental factors that trigger lacustrine radiation can also act in rivers.In general, the main prerequisites for adaptive radiation are: (1) environmental stability for evolutionarily significant periods (starting from thousands of years ago); and (2) physical isolation from the river drainages harboring diverse fish faunas 15,20,31 . The latter serves a dual purpose: first, it provides the ecological opportunities for niche divergence at the initial stages of radiation, and later it prevents the extinction of diverged forms caused by competition with highly specialized species from other systematic groups. These processes can operate in specific and rather rare parts of the riverine network. For example, the segment of the Genale River that harbours the radiating assemblage of distinct forms of the Labeobarbus is situated in the apparently old canyon, isolated by the Baratieri Falls from the lower reaches of the river system and characterized by the depauperate fish fauna 20 .If the fish radiations in rivers are similar to the evolutionary phenomena experienced in lakes, they should display similar evolutionary patterns. It is well known that adaptive radiations are often repeatable in different organisms as exemplified from spiders 34 , fish 35 , and liz...

Monsters with a shortened vertebral column: A population phenomenon in radiating fish Labeobarbus (Cyprinidae)

“…2, Fig 3 ), where most other morphs common to the main sampling site were found. Moreover, the SH morph was not recorded from other sampling sites in the Wabi-Shebele and Juba river systems ( Fig 3 , S2 File ) or from other parts of Ethiopia (sampling sites may be found in [ 35 , 83 , 84 ]). However, at sampling site no.…”

“…The selected fish were maintained for several hours in large barrels submerged in the river, then killed with an overdose of MS-222 and their morphology was preliminarily examined. Tissue samples were taken for genetic and stable isotope analyses because the genetic and trophic specialization studies have been conducted on the same individuals [ 30 , 34 , 35 ]. Most specimens were preserved in 10% formalin in the field, and subsequently transferred to 70% ethanol in the laboratory.…”

“…It is important to note that the barbs inhabiting the southeastern part of Ethiopia (drained by the Wabi-Shebele and Juba river systems into the Indian Ocean) form a monophyletic group, the Labeobarbus gananensis (Vinciguerra, 1895) complex. This is a sister group to Labeobarbus inhabiting enclosed basins of the Ethiopian Rift Valley, as well as all Labeobarbus from the waters of western and northern Ethiopia belonging to the Omo-Turkana and Nile systems, and additional some Kenyan barbs [ 30 , 34 , 35 ].…”

“…In addition to the famed Labeobarbus radiation in Lake Tana [ 36 – 66 ], this group has also experienced parallel riverine radiations in four Ethiopian basins. These riverine radiations are similar in terms of their morphological and ecological differentiation, but differ in the ways of genetic divergence [ 35 ]. Outside of Ethiopia, the riverine Labeobarbus radiation seems to occur in the Inkisi River basin (Lower Congo) [ 67 ].…”

“…It is geographically separated from the other Ethiopian Labeobarbus radiations by the Ethiopian Rift valley. It exhibits greater divergence in morphology, ecology and genetics than other riverine radiations [ 35 ]. During an intensive sampling of the middle Genale River in 2009, we observed a substantial portion of barbs with extremely short and deep bodies [ 34 ].…”