Martin J. Genner

Assortative mating among rock-dwelling cichlid fishes supports high estimates of species richness from Lake Malawi

It has been estimated that Lake Malawi, Africa, contains 500–650 endemic species of cichlid fishes, the largest number of vertebrate species endemic to any comparable sized area on the planet. As many of these putative species cannot be distinguished anatomically, these estimates of species richness depend to a great extent on the assumption that sympatrically occurring male colour morphs represent biological species. We have tested this assumption using a combination of behavioural observations of courtship and microsatellite DNA analysis for six putative species of the Pseudotropheus (Tropheops) complex and three of the Pseudotropheus (Maylandia) complex occurring sympatrically at Nkhata Bay. We were unable to demonstrate assortative courtship for the species pairs Pseudotropheus (Maylandia) zebra/P. ‘gold zebra’ or P. (Tropheops) ‘band’/P. (T.) ‘rust’ because we were unable to distinguish between the females of these taxa. All other taxa showed clear assortative courtship, except for P. (T.) ‘deep’, a deep‐water species which was rarely observed. Fixation indices (θST for the infinite allele model, and RST for the stepwise mutation model) calculated from six microsatellite DNA loci demonstrated significant deviations from panmixia in all pairwise comparisons of putative species, indicating little or no gene flow between populations. All taxa showed high levels of allelic diversity providing evidence that genetic bottlenecking may have been of limited importance in the speciation process. Assortative mating among taxa differing only in male colouration is supportive of theories that speciation in these fishes has been driven by sexual selection by female choice.

Camouflaged invasion of Lake Malawi by an Oriental gastropod.

In this study we report the first animal invasion, to our knowledge, into Lake Malawi. The colonizer is a non‐native morph of the gastropod Melanoides tuberculata that differs substantially in external shell characters from co‐occurring indigenous forms. However, because the species possesses extensive within‐Africa geographical variation in shell morphology, it was unclear whether the invasion was range expansion of a native African morph, or a colonization from elsewhere. Mitochondrial DNA sequences indicate a southeast Asian origin for the invader, suggesting that shell variation found among indigenous allopatric populations camouflaged an intercontinental invasion.

Associations of bird species richness and community composition with local- and landscape-scale environmental factors in Borneo

A comprehensive understanding of variables associated with spatial differences in community composition is essential to explain and predict biodiversity over landscape scales. In this study, spatial patterns of bird diversity in Central Kalimantan, Indonesia, were examined and associated with local-scale (habitat structure and heterogeneity) and landscape-scale (logging, slope position and elevation) environmental variables. Within the study area (c. 196xa0km2) local habitat structure and heterogeneity varied considerably, largely due to logging. In total 9747 individuals of 177 bird species were recorded. Akaikes information criterion (AIC) revealed that the best explanatory models of bird community similarity and species richness included both local- and landscape-scale environmental variables. Important local-scale variables included liana abundance, fern cover, sapling density, tree density, dead wood abundance and tree architecture, while important landscape-scale variables were elevation, logging and slope position. Geographic distance between sampling sites was not significantly associated with spatial variation in either species richness or similarity. These results indicate that deterministic environmental processes, as opposed to dispersal-driven stochastic processes, primarily structure bird assemblages within the spatial scale of this study and confirm that highly variable local habitat measures can be effective means of predicting landscape-scale community patterns.

Amassing diversity in an ancient lake: Evolution of a morphologically diverse parthenogenetic gastropod assemblage in Lake Malawi

Exceptional ecological niche diversity, clear waters and unique divergent selection pressures have often been invoked to explain high morphological and genetic diversity of taxa within ancient lakes. However, it is possible that in some ancient lake taxa high diversity has arisen because these historically stable environments have allowed accumulation of lineages over evolutionary timescales, a process impossible in neighbouring aquatic habitats undergoing desiccation and reflooding. Here we examined the evolution of a unique morphologically diverse assemblage of thiarid gastropods belonging to the Melanoides polymorpha‘complex’ in Lake Malawi. Using mitochondrial DNA sequences, we found this Lake Malawi complex was not monophyletic, instead sharing common ancestry with Melanoides anomala and Melanoides mweruensis from the Congo Basin. Fossil calibrations of molecular divergence placed the origins of this complex to within the last 4 million years. Nuclear amplified fragment length polymorphism markers revealed sympatric M. polymorpha morphs to be strongly genetically differentiated lineages, and males were absent from our samples indicating that reproduction is predominantly parthenogenetic. These results imply the presence of Lake Malawi as a standing water body over the last million years or more has facilitated accumulation of clonal morphological diversity, a process that has not taken place in more transient freshwater habitats. As such, the historical stability of aquatic environments may have been critical in determining present spatial distributions of biodiversity.

Resistance of an invasive gastropod to an indigenous trematode parasite in Lake Malawi

Successful establishment and spread of biological invaders may be promoted by the absence of population-regulating enemies such as pathogens, parasites or predators. This may come about when introduced taxa are missing enemies from their native habitats, or through immunity to enemies within invaded habitats. Here we provide field evidence that trematode parasites are absent in a highly invasive morph of the gastropod Melanoides tuberculata in Lake Malawi, and that the invasive morph is resistant to indigenous trematodes that castrate and induce gigantism in native M. tuberculata. Since helminth infections can strongly influence host population abundances in other host-parasite systems, this enemy release may have provided an advantage to the invasive morph in terms of reproductive capacity and survivorship.

Reproductive isolation among deep‐water cichlid fishes of Lake Malawi differing in monochromatic male breeding dress

Male nuptial colour hues are important for the maintenance of reproductive isolation among cichlid fish species, and environmental changes that lead to narrower light spectra can lead to hybridization. However, cichlid species can naturally co‐occur in narrow light spectrum habitats, such as turbid shallow lakes and the deep benthic zones of African rift lakes. Closely related species from narrow light spectrum habitats tend to differ little in the palette of male nuptial colours, thus for these taxa differences in colour patterns may be more important than differences in colour hue for species recognition. To investigate this hypothesis we examined morphometric and genetic differentiation among males of four sympatric putative species within the deep‐water genus Diplotaxodon. These taxa live in a narrow‐light spectrum environment where only blue light is present, and males differ primarily in ‘monochromatic’ black, white and silver patterning of the body and fins. Significant genetic differentiation was present among taxa in both microsatellite DNA and mitochondrial DNA, including one pair with no significant morphometric differentiation. Thus, these taxa represent reproductively isolated biological species, a result consistent with male nuptial patterning being important for species recognition and assortative mating. As such, we suggest that narrow‐light spectra need not always represent barriers to effective visually mediated mate recognition.

Secondary contact seeds phenotypic novelty in cichlid fishes.

Theory proposes that genomic admixture between formerly reproductively isolated populations can generate phenotypic novelty for selection to act upon. Secondary contact may therefore be a significant promoter of phenotypic novelty that allows species to overcome environmental challenges and adapt to novel environments, including during adaptive radiation. To date, this has largely been considered from the perspective of interspecific hybridization at contact zones. However, it is also possible that this process occurs more commonly between natural populations of a single species, and thus its importance in adaptive evolution may have been underestimated. In this study, we tested the consequences of genomic introgression during apparent secondary contact between phenotypically similar lineages of the riverine cichlid fish Astatotilapia calliptera. We provide population genetic evidence of a secondary contact zone in the wild, and then demonstrate using mate-choice experiments that both lineages can reproduce together successfully in laboratory conditions. Finally, we show that genomically admixed individuals display extreme phenotypes not observed in the parental lineages. Collectively, the evidence shows that secondary contact can drive the evolution of phenotypic novelty, suggesting that pulses of secondary contact may repeatedly seed genetic novelty, which when coupled with ecological opportunity could promote rapid adaptive evolution in natural circumstances.

Diversity patterns of Bornean butterfly assemblages

Borneo contains a diverse rainforest butterfly community, but its forests are under threat from logging and ENSO- (El Niño Southern Oscillation) induced fires. Contrasts in butterfly assemblage structure were examined in nine 450 ha landscapes in logged forest, primary unburned continuous and isolated forest, and forest affected by surface fires during the 1997/98 ENSO event. Temporally the effect of the 1997/98 ENSO event was followed in a single burned landscape from 1997 to 2004. In total, 517 species were present in 190 sampling sites. There was a five-fold difference in species richness among landscapes, with highest richness in continuous landscapes and lowest richness in burned landscapes. Richness was also higher in logged forest than proximate unlogged forest. Temporally, species richness dropped dramatically from 1997 to 1998, but afterwards increased remaining, however, substantially lower than pre-ENSO (1997) sampling. Sites in burned landscapes were distinct from other sites in terms of vegetation structure with the slash-and-burn area the most dissimilar to other landscapes. There was much less structure among unburned landscapes. The pattern of butterfly community composition was similar to that of vegetation structure with the community from the slash-and-burn area the most distinct. However, there was much less overlap among sites from different landscapes. Temporally, 1998 possessed the most distinct assemblage when compared to assemblages from other years. The community composition was, however, slowly returning to a pre-disturbance composition. Variance in community composition explained by environmental and spatial factors differed substantially among landscapes. The spatial fraction was the only explanatory component in recently burned landscapes and a proximate small unburned isolate, but explained no variation in logged landscapes. The environmental fraction explained substantial amounts of variation in logged landscapes and the slash-and-burn area. When all landscapes were pooled high proportions of variation in butterfly community composition were explained by both geographic distance between sites and environmental variables. In contrast when only unburned landscapes were considered, most variation was explained by the geographic distance among them. Despite differences among landscapes there was a general pattern of relatively sharp decline in similarity at short distances that levels out over greater distances, a result that agrees with previous studies on other tropical species assemblages.