* assessed by mtDNA only.
The cases of divergence between sympatric ecomorphs in various rivers
are ranked from 1 to 3 points. The results can be treated as a row of
situations from ecologically non-functional polymorphism (1 point)via ecologically functional polymorphism (2 points) to divergent
evolution or completed speciation (3 points in the presence of genetic
divergence). The case of the Sore and Gojeb (2 points) showing the
morphological and trophic divergence but no genome-wide divergence can
be considered an initial stage of speciation with only few loci
involved. Studies on genomic differentiation of thick-lipped phenotype
in Amphilophus cichlids from Nicaraguan lakes showed that only a
small set of loci are responsible for this phenotype (Kautt et al.,
2012; Machcado-Schiaffino et al., 2017; Kautt et al., 2020; Sowersby et
al., 2021). As for a sequence of divergence events, the evolution of
thick-lipped species of Labeobarbus might be realized into three
steps: i) phenotypic divergence, ii) ecological (trophic) divergence,
iii) genetic divergence. At the last stage when all three types of
divergence (phenotypic, ecological, and genetic) begin to be detectable
a species status is achieved. Accordingly, the sympatric pair from the
Didessa River can be considered two different species. Notable
divergence in population genomic structure based on ddRAD SNPs in the
lack of sorting in mtDNA evidences for a rather recent speciation in the
case of the Didessa River radiation.
We detected a gradual increase of divergence among the population pairs
evolving in parallel. Similar situation of the sympatric differentiation
is reported for the cichlid genus Amphilophus which demonstrates
a speciation continuum from incipient to fully completed (Torres-Dowdall
& Meyer, 2021; Sowersby et al., 2021). These examples correspond to the
concept of speciation continuum that is considered a speciation as a
continuum of stages of reproductive (and other) isolations (Drès &
Mallet, 2002; Stankowski & Ravinet, 2021; Bolnick et al., 2023). It is
noteworthy that such a continuum is found in Labeobarbuspopulations inhabiting the rivers while all previous examples (among
fishes) came from the lacustrine environment. Generally, ecological
speciation is more common among fishes under the lacustrine conditions
(reviewed in Seehausen & Wagner, 2014). However, the Ethiopian
cyprinids (Levin et al., 2019; 2020; 2021a,b; Golubtsov et al., 2021)
and South American cichlids (Burress et al., 2018), as well as the
Kamchatka salmonids (Esin et al., 2021, 2022), demonstrate this
evolutionary pattern in rivers.
For a better understanding of the nature of numerously proliferated
polymorphisms of the mouth structure in Labeobarbus we have to
study the evolutionary history of the Torinae lineages in the future.
Mouth polymorphisms in relation to lip size including state
‘hyperthophied lips’ is common within the closely-related polyploid
cyprinid lineages from the Middle East (e.g., ArabibarbusBorkenhagen, 2014 and Carasobarbus Karaman, 1971). This
polymorphism is probably ancestral to both African and Middle East
Torinae being inherited from Southern and Southeastern tetraploid
Torinae lineages (e.g., Tor Gray, 1834 and NeolissochilusRainboth, 1985 – Roberts & Khaironizam, 2008; Yang et. al, 2015,
2022). Thus, a proliferation of a repeated, and predicted thick-lipped
phenotype within Labeobarbus lineage might be due to an ancient
genetic polymorphism that re-evolved under particular ecological
circumstances. Given this, the thick-lipped phenotype ofLabeobarbus is obviously “pre-adaptive” upon emergencede-novo in various populations but not always functional, i.e.,
not yet necessarily involved in trophic resource partitioning according
to our results (Figures 3B and 4). Nevertheless, prevalence of
invertebrates in a diet or elevated d15N values in the thick-lipped
ecomorph were discovered in all cases (including non-significant
differences). This finding may be interpreted as a result of the
generally adaptive value of the hypertrophied lips. In those populations
that could not demonstrate an adaptive value of the hypertrophied lips
in terms of trophic resource partitioning, the thickened lips might
serve as a latent “adaptation in reserve” or “silent adaptation”
that can be recruited into invertebrate foraging at appropriate
environmental conditions possible due to being maintained in the
population in low frequency and due to simple genetic and developmental
programs that might also be facilitated by genetic assimilation based on
initially phenotypic plasticity (Meyer, 1987; Schneider et al., 2014;
Gunter et al., 2017).