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The African weakly electric fish genus Campylomormyrus includes 15 described species mostly native to the Congo River and its tributaries. They are considered sympatric species, because their distribution area overlaps. These species generate species-specific electric organ discharges (EODs) varying in waveform characteristics, including duration, polarity, and phase number. They exhibit also pronounced divergence in their snout, i.e. the length, thickness, and curvature. The diversifications in these two phenotypical traits (EOD and snout) have been proposed as key factors promoting adaptive radiation in Campylomormyrus. The role of EODs as a pre-zygotic isolation mechanism driving sympatric speciation by promoting assortative mating has been examined using behavioral, genetical, and histological approaches. However, the evolutionary effects of the snout morphology and its link to species divergence have not been closely examined. Hence, the main objective of this study is to investigate the effect of snout morphology diversification and its correlated EOD to better understand their sympatric speciation and evolutionary drivers. Moreover, I aim to utilize the intragenus and intergenus hybrids of Campylomormyrus to better understand trait divergence as well as underlying molecular/genetic mechanisms involved in the radiation scenario. To this end, I utilized three different approaches: feeding behavior analysis, diet assessment, and geometric morphometrics analysis. I performed feeding behavior experiments to evaluate the concept of the phenotype-environment correlation by testing whether Campylomormyrus species show substrate preferences. The behavioral experiments showed that the short snout species exhibits preference to sandy substrate, the long snout species prefers a stone substrate, and the species with intermediate snout size does not exhibit any substrate preference. The experiments suggest that the diverse feeding apparatus in the genus Campylomormyrus may have evolved in adaptation to their microhabitats. I also performed diet assessments of sympatric Campylomormyrus species and a sister genus species (Gnathonemus petersii) with markedly different snout morphologies and EOD using NGS-based DNA metabarcoding of their stomach contents. The diet of each species was documented showing that aquatic insects such as dipterans, coleopterans and trichopterans represent the major diet component. The results showed also that all species are able to exploit diverse food niches in their habitats. However, comparing the diet overlap indices showed that different snout morphologies and the associated divergence in the EOD translated into different prey spectra. These results further support the idea that the EOD could be a ‘magic trait’ triggering both adaptation and reproductive isolation. Geometric morphometrics method was also used to compare the phenotypical shape traits of the F1 intragenus (Campylomormyrus) and intergenus (Campylomormyrus species and Gnathonemus petersii) hybrids relative to their parents. The hybrids of these species were well separated based on the morphological traits, however the hybrid phenotypic traits were closer to the short-snouted species. In addition, the likelihood that the short snout expressed in the hybrids increases with increasing the genetic distance of the parental species. The results confirmed that additive effects produce intermediate phenotypes in F1-hybrids. It seems, therefore, that morphological shape traits in hybrids, unlike the physiological traits, were not expressed straightforward.
Weakly electric mormyrid fish comprise about 200 species. 15 species of the genus Campylomormyrus have been described. These are very diverse concerning the trunk-like snout and the shape and duration of the electric organ discharge (EOD) and the anatomy of the electric organ. In this dissertation data on the reproduction in captivity of four species and on the ontogeny of the EOD and the EO of three species are presented.
Reproduction of the four species C. compressirostris, C. rhynchophorus, C. tshokwe and C. numenius: Cyclical reproduction was provoked by changing only water conductivity (C): decreasing C led to gonadal recrudescence, an increase induced gonad regression. Data on the reproduction and development of three species are presented (in C. numenius gonad development could only be achieved in males). Agonistic behavior in the C. tshokwe pair forced us to divide the breeding tank; therefore, only ovipositions occurred. However, injection of an artificial GnRH hormone allowed us to obtain ripe eggs and sperm and to perform successful artificial reproduction. All three species (C. compressirostris, C. rhynchophorus, C. tshokwe) are indeterminate fractional spawners. Spawnings/ovipositions occurred during the second half of the night; no parental care was observed; no special spawning substrates were necessary. C. compressirostris successfully spawned in breeding groups, C. rhynchophorus as pair. Spawning intervals ranged from 6 to 66 days in C. rhynchophorus, 10–75 days in C. tshokwe, and 18 days in C. compressirostris (calculated values). Fecundities (eggs per fractional spawning) ranged from 70 to 1570 eggs in C. rhynchophorus, 100–1192 in C. tshokwe, and 38–246 in C. compressirostris. All three species produce yolky, slightly sticky eggs. Egg diameter ranges from 2.3–3.0 mm. Hatching occurred on day 3, feeding started on day 11. Transition from larval to juvenile stage occurred at around 20 mm total length (TL). At this size C. rhynchophorus developed a higher body than the two other species and differences between the species in the melanin pigmentation of the unpaired fins occurred. Between 32 and 35 mm TL the upper and lower jaws developed.
C. compressirostris and C. tamandua are similar in morphology and both produce short EODs of ca. 150-200 μs duration. Both species reproduce easily in captivity. We tried to obtain natural hybrids in two breeding groups, 1) four males of C. compressirostris and three females of C. tamandua and 2) six females of C. compressirostris and four males of C. tamandua. In both combinations several times oviposition occurred, however, we never found fertilized eggs. In subsequent experiments, not described here, we obtained hybrids between these two species by means of artificial reproduction.
Ontogeny of the EOD and the EO: The Campylomormyrus species are very diverse both concerning the shape and the duration of their EODs. There are species with very short EODs, e.g. C. compressirostris duration, a species with an EOD length of about 4-8 ms duration (C. tshokwe) and species with very long EODs of about 25 ms duration (e.g. C. rhynchophorus). Due to the successful breeding of the three species in captivity, we were able to investigate in detail the ontogeny of the EOD. Larvae of the three species C. compressirostris, C. tshokwe and C. rhynchophorus first produce a biphasic larval EOD typical for these small larvae. The first activity of the adult electric organ in the caudal peduncle is a biphasic juvenile EOD. Juvenile C. compressirostris and C. tshokwe start out with a short biphasic EOD of about 160 – 200 μs duration at sizes between 25 mm (C. compressirostris) and 37 mm (C. tshokwe). Adult C. compressirostris show an EOD identical to that of the juvenile. In C. tshokwe, the juvenile EOD changes continuously during development both concerning duration, amplitude increase and shape. 18 cm long C. tshokwe still do not yet produce an EOD typical for the adult fish. Juveniles of C. rhynchophorus produce at 33 mm total length a juvenile biphasic EOD, however, of longer duration (about 640 μs) than the two species mentioned above. This juvenile EOD changes continuously both in form, amplitude increase and duration with growth until the adult EOD waveform appears at about 15 cm body length. In juveniles about seven cm long the triphasic feature of the EOD starts to develop due to the appearance of a second head positive phase. Specific EOD stages are produced in relation to size and not to age. Individual differences in the EOD both concerning shape and duration are very small. The basic anatomy of the electrocytes is very similar in all three species: the main stalk which receives the innervation, is located at the caudal face of the electrocyte. Membrane penetrations of the stalks do not occur. However, there are differences in the fine structure of the electrocytes in the three species. Papillae, proliferations of the membrane, which increase the surface area of the electrocyte and are thought to incrase the EOD-duration, are only found in C. tshokwe and C. rhynchophorus. In these two species in addition, holes develop in the electrocytes during ontogeny. This might also have an impact on EOD duration.
The African weakly electric fish genus Campylomormyrus is a well-investigated fish group of the species-rich family Mormyridae. They are able to generate species-specific electric organ discharges (EODs) which vary in their waveform characteristics including polarity, phase umber and duration. In mormyrid species EODs are used for communication, species discrimination and mate recognition, and it is thought hat they serve as pre-zygotic isolation mechanism driving sympatric speciation by promoting assortative mating. The EOD diversification, its volutionary effects and the link to species divergence have been examined histologically, behaviorally, and genetically. Molecular analyses are a major tool to identify species and their phenotypic traits by studying the underlying genes. The genetic variability between species further provides information from which evolutionary processes, such as speciation, can be deduced. Hence, the ultimate aim of this study is the investigation of genetic variability within the African weakly electric fish genus Campylomormyrus to better understand their sympatric speciation and comprehend their evolutionary drivers. In order to extend the current knowledge and gain more insights into its species history, karyological and genomic approaches are being pursued considering species differences. Previous studies have shown that species with different EOD duration have specific gene expression patterns and single nucleotide polymorphisms (SNPs). As EODs play a crucial role during the evolution of Campylomormyrus species, the identification of its underlying genes may suggest how the EOD diversity evolved and whether this trait is based on a complex network of genetic processes or is regulated by only a few genes. The results obtained in this study suggest that genes with non-synonymous SNPs, which are exclusive to C. tshokwe with an elongated EOD, have frequent functions ssociated with tissue morphogenesis and transcriptional regulation. Therefore, it is proposed that these processes likely co-determine EOD characteristics of Campylomormyrus species. Furthermore, genome-wide analyses confirm the genetic difference among most Campylomormyrus species. In contrast, the same analyses reveal genetic similarity among individuals of the alces-complex showing different EOD waveforms. It is therefore hypothesized that the low genetic variability and high EOD diversity represents incipient sympatric speciation. The karyological description of a Campylomormyrus species provides crucial information about chromosome number and shapes. Its diploid chromosome number of 2n=48 supports the conservation of this trait within Mormyridae. Differences have been detected in the number of bi-armed chromosomes which is unusually high compared to other mormyrid species. This high amount can be due to chromosome rearrangements which could cause genetic incompatibility and reproductive isolation. Hence an alternative hypothesis regarding processes which cause sympatric speciation is that chromosome differences are involved in the speciation process of Campylomormyrus by acting as postzygotic isolation mechanism. In summary, the karyological and genomic investigations conducted in this study contributed to the increase of knowledge about Campylomormyrus species, to the solution of some existing ambiguities like phylogenetic relationships and to the raising of new hypothesis explaining the sympatric speciation of those African weakly electric fish. This study provides a basis for future genomic research to obtain a complete picture for causes and results of evolutionary processes in Campylomormyrus.
