Mairi E. Knight

How many species of cichlid fishes are there in African lakes

The endemic cichlid fishes of Lakes Malawi, Tanganyika and Victoria are textbook examples of explosive speciation and adaptive radiation, and their study promises to yield important insights into these processes. Accurate estimates of species richness of lineages in these lakes, and elsewhere, will be a necessary prerequisite for a thorough comparative analysis of the intrinsic and extrinsic factors influencing rates of diversification. This review presents recent findings on the discoveries of new species and species flocks and critically appraises the relevant evidence on species richness from recent studies of polymorphism and assortative mating, generally using behavioural and molecular methods. Within the haplochromines, the most species‐rich lineage, there are few reported cases of postzygotic isolation, and these are generally among allopatric taxa that are likely to have diverged a relatively long time in the past. However, many taxa, including many which occur sympatrically and do not interbreed in nature, produce viable, fertile hybrids. Prezygotic barriers are more important, and persist in laboratory conditions in which environmental factors have been controlled, indicating the primary importance of direct mate preferences. Studies to date indicate that estimates of alpha (within‐site) diversity appear to be robust. Although within‐species colour polymorphisms are common, these have been taken into account in previous estimates of species richness. However, overall estimates of species richness in Lakes Malawi and Victoria are heavily dependent on the assignation of species status to allopatric populations differing in male colour. Appropriate methods for testing the specific status of allopatric cichlid taxa are reviewed and preliminary results presented.

An interspecific comparison of foraging range and nest density of four bumblebee (Bombus) species

Bumblebees are major pollinators of crops and wildflowers in northern temperate regions. Knowledge of their ecology is vital for the design of effective management and conservation strategies but key aspects remain poorly understood. Here we employed microsatellite markers to estimate and compare foraging range and nest density among four UK species: Bombus terrestris, Bombus pascuorum, Bombus lapidarius, and Bombus pratorum. Workers were sampled along a 1.5‐km linear transect across arable farmland. Eight or nine polymorphic microsatellite markers were then used to identify putative sisters. In accordance with previous studies, minimum estimated maximum foraging range was greatest for B. terrestris (758 m) and least for B. pascuorum (449 m). The estimate for B. lapidarius was similar to B. pascuorum (450 m), while that of B. pratorum was intermediate (674 m). Since the area of forage available to bees increases as the square of foraging range, these differences correspond to a threefold variation in the area used by bumblebee nests of different species. Possible explanations for these differences are discussed. Estimates for nest density at the times of sampling were 29, 68, 117, and 26/km2 for B. terrestris, B. pascuorum, B. lapidarius and B. pratorum, respectively. These data suggest that even among the most common British bumblebee species, significant differences in fundamental aspects of their ecology exist, a finding that should be reflected in management and conservation strategies.

Divergent selection during speciation of Lake Malawi cichlid fishes inferred from parallel radiations in nuptial coloration

Repeated evolution of the same phenotypic difference during independent episodes of speciation is strong evidence for selection during speciation. More than 1,000 species of cichlids, >10% of the worlds freshwater fish species, have arisen within the past million years in Lakes Malawi and Victoria in eastern Africa. Many pairs of closely related sympatric species differ in their nuptial coloration in very similar ways. Nuptial coloration is important in their mate choice, and speciation by sexual selection on genetically or ecologically constrained variation in nuptial coloration had been proposed, which would repeatedly produce similar nuptial types in different populations, a prediction that was difficult to test in the absence of population-level phylogenies. We measured genetic similarity between individuals within and between populations, species, and lake regions by typing 59 individuals at >2,000 polymorphic genetic loci. From these data, we reconstructed, to our knowledge, the first larger species level phylogeny for the most diverse group of Lake Malawi cichlids. We used the genetic and phylogenetic data to test the divergent selection scenario against colonization, character displacement, and hybridization scenarios that could also explain diverse communities. Diversity has arisen by replicated radiations into the same color types, resulting in phenotypically very different, yet closely related, species within and phenotypically highly similar yet unrelated sets of species between regions, which is consistent with divergent selection during speciation and is inconsistent with colonization and character displacement models.

Nuclear markers reveal unexpected genetic variation and a Congolese-Nilotic origin of the Lake Victoria cichlid species flock

Phylogenetic analyses based on mitochondrial (mt) DNA have indicated that the cichlid species flock of the Lake Victoria region is derived from a single ancestral species found in East African rivers, closely related to the ancestor of the Lake Malawi cichlid species flock. The Lake Victoria flock contains ten times less mtDNA variation than the Lake Malawi radiation, consistent with current estimates of the ages of the lakes. We present results of a phylogenetic investigation using nuclear (amplified fragment length polymorphism) markers and a wider coverage of riverine haplochromines. We demonstrate that the Lake Victoria–Edward flock is derived from the morphologically and ecologically diverse cichlid genus Thoracochromis from the Congo and Nile, rather than from the phenotypically conservative East African Astatotilapia. This implies that the ability to express much of the morphological diversity found in the species flock may by far pre–date the origin of the flock. Our data indicate that the nuclear diversity of the Lake Victoria–Edward species flock is similar to that of the Lake Malawi flock, indicating that the genetic diversity is considerably older than the 15 000 years that have passed since the lake began to refill. Most of this variation is manifested in trans–species polymorphisms, indicating very recent cladogenesis from a genetically very diverse founder stock. Our data do not confirm strict monophyly of either of the species flocks, but raise the possibility that these flocks have arisen from hybrid swarms.

Laboratory mating trials indicate incipient speciation by sexual selection among populations of the cichlid fish Pseudotropheus zebra from Lake Malawi.

It has been suggested that sexual selection may have played a major role in the rapid evolution of hundreds of species of cichlid fishes in Lake Malawi. We report the results of a laboratory test of assortative mating among Lake Malawi cichlid fishes from five closely related geographical populations differing in male courtship colour. Paternity of clutches was tested using microsatellite DNA typing of offspring. Out of 1955 offspring typed, 1296 (66.3%) were sired by the male from the same population as the female, which is more than three times the rate expected if females do not differentiate among males of the different populations (20%). This result indicates that mate preferences of geographical races are strongly differentiated, consistent with the races representing incipient geographical species diverging under sexual selection exerted by female preferences for different male courtship colours.

Extremely low effective population sizes, genetic structuring and reduced genetic diversity in a threatened bumblebee species, Bombus sylvarum (Hymenoptera: Apidae)

Habitat fragmentation may severely affect survival of social insect populations as the number of nests per population, not the number of individuals, represents population size, hence they may be particularly prone to loss of genetic diversity. Erosion of genetic diversity may be particularly significant among social Hymenoptera such as bumblebees (Bombus spp.), as this group may be susceptible to diploid male production, a suggested direct cost of inbreeding. Here, for the first time, we assess genetic diversity and population structuring of a threatened bumblebee species (Bombus sylvarum) which exists in highly fragmented habitat (rather than oceanic) islands. Effective population sizes, estimated from identified sisterhoods, were very low (range 21–72) suggesting that isolated populations will be vulnerable to loss of genetic variation through drift. Evidence of significant genetic structuring between populations (θ = 0.084) was found, but evidence of a bottleneck was detected in only one population. Comparison across highly fragmented UK populations and a continental population (where this species is more widespread) revealed significant differences in allelic richness attributable to a high degree of genetic diversity in the continental population. While not directly related to population size, this is perhaps explained by the high degree of isolation between UK populations relative to continental populations. We suggest that populations now existing on isolated habitat islands were probably linked by stepping‐stone populations prior to recent habitat loss.

Evidence for male-biased dispersal in lake malawi cichlids from microsatellites

This study addressed within‐population dispersal patterns among the species‐rich Lake Malawi cichlids, specifically among the rock‐dwelling mbuna group. Relatedness values were calculated for 160 individuals belonging to two species from known locations in the field by screening six highly polymorphic microsatellite loci. The results from both species indicate that spatially adjacent females have higher average relatedness values than those separated by larger distances, but that this pattern is reversed in males. This therefore provides firm evidence for male‐biased dispersal within the Malawian cichlid flock.

Microsatellite paternity analysis on captive Lake Malawi cichlids supports reproductive isolation by direct mate choice

Three sympatrically occurring members of a cichlid fish species complex from Lake Malawi have previously been reported to show assortative mating in the field. Offspring from three experimental tanks each housing both sexes of all three members of this complex were screened at one to four microsatellite loci to determine paternity. Reproductive isolation was found to persist without exception under laboratory conditions, providing strong evidence for behavioural barriers to mating. Direct mate choice is suggested as the process preventing interbreeding. The results are interpreted with reference to the rapid and explosive evolution of the cichlid group.

Bumblebee nest density and the scale of available forage in arable landscapes

Abstract. 1 Combining the needs of agricultural production with enhancing biodiversity requires a landscape‐scale approach since the geographic scale at which most non‐farmed species operate is unconstrained by farm boundaries. Bumblebees are a key component of farmland biodiversity as pollinators of both crops and wild flora. However, the factors determining their densities in such landscapes remain poorly understood. 2 Using a combination of remote‐sensed landscape data and molecular markers, we quantify the effects of land use (oilseed rape, field beans and non‐cropped areas, all of which provide suitable bumblebee forage), at various spatial scales to find the best predictor of colony density for the bumblebee Bombus pascuorum Scopoli in an arable landscape. 3 Estimated colony density was positively correlated with the area of all habitat categories within 1000 m of the sample site. No significant relationships were found for greater or lesser distances. This concurs with earlier estimates of the foraging range of this species. We found no evidence that nest sizes increased with forage availability, although our data do not allow us to categorically exclude this possibility. 4 It has long been suspected that forage availability limits bee abundance in agricultural landscapes but there is little direct evidence for this. Here we report a direct relationship between floral abundance and bumblebee nest density within a notionally fixed area. Importantly, we suggest that the forage availability within the previously published estimated foraging distance for this species at this location is a good predictor of the scale of impact of forage provision on nesting density.

Reproductive isolation among closely related Lake Malawi cichlids: can males recognize conspecific females by visual cues?

Little is known about mate recognition systems among the species-rich cichlid flocks of the African Great Lakes. Such knowledge is critical to our understanding of how evolutionary processes have brought about the current diversity. In the present study we focused on three anatomically similar species of rocky shore cichlid fish Pseudotropheus (Maylandia) spp. from Lake Malawi. We scored male responses to heterospecific and conspecific females which were enclosed in separate watertight transparent chambers. Any male behaviour we observed was therefore due to visual cues alone. Although males of some species could clearly identify conspecific females on visual cues alone, others could not. In cases where conspecific and heterospecific females were phenotypically similar, males could not consistently distinguish between them. Further experiments are required, however, to see whether males can recognize these females using alternative means, such as olfactory cues. Copyright 1999 The Association for the Study of Animal Behaviour.