Introduction
Phenotypic variation provides the crucial basis for divergent selection to act upon and is the source for further evolution of novel morphological and ecological diversity. Variation in trophic morphology is of particular interest to evolutionary biologists since the partitioning of trophic resources is considered one of the main prerequisites for ecological speciation and adaptive radiation (Meyer, 1987; Sibbing et al., 1998; Rüber et al., 1999; Rundle et al., 2000). Linking phenotypic traits to an adaptive ecological function requires information at different levels of biological organization. Moreover, it remains uncertain what is primary during adaptive radiation – behavioral or ecological adaptation that precedes morphological specialization or morphological novelty promoting filling of the new ecological niches (reviewed in Schluter, 2000). Adaptive radiations based on trophic resource partitioning have been studied in fishes such as cichlids, coregonids, Arctic charr, three-spined sticklebacks, and many other lineages of fish (Skulason, Smith, 1995; Schluter, 2000; Sibbing, Nagelkerke, 2000; Barluenga et al., 2006; Martin and Wainwright, 2011; Seehausen and Wagner, 2014; Burress, 2016). Cyprinid fishes, Family Cyprinidae sensu lato , is one of the most diversified families of Actinopterygii (> 3000 species – Fricke et al., 2022) that contains many adaptive radiations (e.g., Kornfield, Karpenter, 1984; Savvaitova et al., 1987; Nagelkerke et al., 1994; Mina et al., 1996; Levin et al., 2020; 2021a; Komarova et al., 2021), most of which, however, have not yet been studied using genome-wide approaches.
Large African barbs of the genus Labeobarbus Rüppell, 1835 are a remarkably diverse lineage of polyploid cyprinid fishes (2n=150 – Oellermann, Skelton, 1990; Golubtsov, Krysanov, 1993) with > 130 species (Fricke, 2022) that display great diversity as well as the distinct polymorphisms in mouth phenotypes (Banister, 1973). This diversity in trophic morphology is part of the explanation for why in the genus Labeobarbus numerous adaptive radiations based on tropic resource partitioning evolved. Such radiations are found in both lacustrine (e.g., the Lake Tana radiation is composed of 15 ecomorphs/species) as well as riverine environments (Nagelkerke et al., 1994; Mina et al., 1996; Levin et al., 2019; 2020; Mironovsky et al., 2019). Four major trophic phenotypes among Labeobarbus have been described: i) generalized; ii) algae scraping (with subtypes); iii) with hypertrophied lips (thick-lipped); and iv) large-mouthed, or piscivorous (with subtypes) (Banister, 1973; Nagelkerke et al., 1994; Mina et al., 1998; Vreven et al., 2016; Levin et al., 2020). Remarkably, the very similar mouth phenotypes repeatedly evolved throughout the range ofLabeobarbus in Sub-Saharan Africa (Tsigenopoulos et al., 2010; Levin et al., 2013; 2020; Vreven et al., 2016). One of the trophic types – the thick-lipped phenotype – is of particular interest to us and is characterized by hypertrophied lips with a well-developed fleshy lobe on the lower jaw, which looks like a fleshy appendage on the chin (Fig. 1 and 2B). The feeding preferences of fishes with such hypertrophied lips was most comprehensively studied in cichlids of both African and American clades (e.g., Ribbink et al., 1983; Stiassny and Meyer 1999; Colombo et al., 2012; Manousaki et al., 2013; Baumgarten et al., 2015; Machado-Schiaffino et al. 2017; Torres-Dowdall & Meyer, 2021; Sowersby et al., 2021). The cichlid species or ecomorphs with hypertrophied lips (also known as “rubberlip”) display also other adaptive features (like narrow and pointed head shape etc. – see also Franchini et al. 2014; Fruciano et al. 2016) evolved a new foraging strategy in rocky crevices thereby occupying a new ecological niche. In particular, hypertrophied lips might increase suction power by sealing cracks and grooves and also protect the head from injury from prey on hidden benthic organisms (Oliver & Arnegard, 2010; Baumgarten et al., 2015). Thickened lips may also allow these cichlid species to achieve higher numbers of taste receptors (Schemmel, 1967; Oliver & Arnegard, 2010; Manousaki et al. 2013).
The thick-lipped mouth phenotype is one of the most frequently occurring within the Labeobarbus lineage (Banister, 1973; Vreven et al., 2016). Some individuals have such greatly hypertrophied lips that pioneering investigator E. Rüppell assigned a new generic name to them based on this conspicuous trophic feature (Rüppell, 1835). Genetic data (mtDNA) support numerous parallel origins of the thick-lipped phenotype among Labeobarbus (Tsigenopoulos et al., 2010; Levin et al., 2020; Decru et al., 2022). Nevertheless, the ecological role of the thick-lipped ecomorphs in Labeobarbus is not yet known. The feeding preferences or trophic position of the thick-lipped ecomorph was investigated so far for two cases only. One study (Sibbing & Nagelkerke, 2001) investigated the diet composition of the lacustrine lipped phenotype of L. cf. intermedius (known also asL. nedgia Rüppell, 1835) in Lake Tana (East Africa, Ethiopia). Another study (Levin et al., 2019) examined nitrogen and carbon stable isotope signatures in riverine lipped phenotype of L. gananensisfrom the Genale River (Ethiopia). Their results provided weak support for trophic resource partitioning between fish with hypertrophied and normally developed lips. So weak trophic resource partitioning between sympatrically co-occurring ecomorphs with strikingly divergent trophic morphology turned out to be an intriguing finding. In general, whether such mouth structure transition is just non-adaptive phenotypic variation within highly polymorphic Labeobarbus lineage or thick lips are an adaptive trait involved in trophic resource partitioning remains unclear so far. To address this question, similar cases in a broader taxonomic and geographical context have been studied by us. We could show that thick-lipped ecomorphs evolved several times independently in several Ethiopian river basins. Yet, paradoxically, the hypertrophied ecomorphs were indistinguishable by mtDNA from sympatric ecomorphs with a generalized mouth phenotype (Levin et al., 2020).
Our goals were two-fold. First, we aimed to test whether hypertrophied lips are adaptive within Labeobarbus as judged by information on trophic resource partitioning, i.e., whether a thick-lipped ecomorph occupies a separate trophic niche or whether these trophic variants might be explained best by phenotypic plasticity alone. To do this, we compared the trophic morphology, diet and stable isotopes signatures in generalized and thick-lipped forms. Second, we tested the hypotheses about genetic divergence between and reproductive isolation within sympatric pairs of thick-lipped and generalized small-lipped ecomorphs based on genome-wide genetic data obtained by double digest restriction-site associated DNA (ddRAD).