K. M. Ibrahim

Spatial patterns of genetic variation generated by different forms of dispersal during range expansion

We examined the impact of three forms of dispersal, stepping-stone, normal and leptokurtic, on spatial genetic structure of expanding populations using computer simulations. When dispersal beyond neighbouring demes is allowed, rare long-distance migration leads to the establishment of pocket populations in advance of the main invasion front and results in spatial clustering of genotypes which persists for hundreds of generations. Patchiness is more pronounced when dispersal is leptokurtic as is the case in many animal and plant species. These results are of particular interest because population genetic parameters such as gene flow and effective population size are commonly estimated using gene frequency divergence information assuming equilibrium conditions and island models. We show how the three forms of dispersal during colonization bring about contrasting population genetic structures and how this affects estimates of gene flow. The implications for experimental studies of the spatial dimension of population genetic structure are discussed.

Postglacial expansion and genome subdivision in the European grasshopper Chorthippus parallelus

A noncoding nuclear DNA marker sequence (Cpnl‐1) was used to investigate subdivision in the grasshopper Chorthippus parallelus and deduce postglacial expansion patterns across its species range in Europe. Investigation of the spatial distribution of 71 Cpnl‐1 haplotypes and estimation of levels of genetic differentiation (KST values) between populations and geographic regions provided evidence for subdivision of C. parallelus into at least five major geographic regions and indicated that the French form of C. parallelus originated after range expansion from a Balkan refugium, Further evidence for subdivision of C. parallelus between Italy and northern Europe suggests that the Alps may have formed a significant barrier to gene flow in this grasshopper.

mtDNA phylogeography and postglacial patterns of subdivision in the meadow grasshopper Chorthippus parallelus

A 300 bp portion of the mitochondrial cytochrome oxidase subunit I (COI) gene was used to investigate the intraspecific genetic structure of the European meadow grasshopper Chorthippus parallelus. Levels of genetic subdivision between geographical regions in Europe were assessed using the KST statistic and revealed patterns indicative of the postglacial history of this organism. Northern European populations are shown to share a very recent common ancestor with, and hence originate from, a Balkan expansion. Genetic distances between these areas and both southern Italy and central Spain are shown to be approximately equal, even though only the Spanish form is recognized as a distinct subspecies. Distance estimates indicate that the common ancestor of the two subspecies lived more than one glacial cycle ago, and the refugial populations have probably been diverging in isolation for five or six glacial cycles (≈550 000 years). Comparisons are made to the analysis of a similar data set using an anonymous nuclear DNA marker (Cooper et al., 1995). Although the structure revealed is generally very similar, differences in the location of transition zones between different genetic forms of C. parallelus may reflect the different historical assortment of these molecules.

Fine-scale genetic structuring in a natural population of European wild rabbits (Oryctolagus cuniculus)

The genetic structure of a free‐living tagged population of European wild rabbits (Oryctolagus cuniculus) was investigated for two consecutive years (1990 and 1991) using 10 polymorphic microsatellite loci. A specific social behaviour, the formation of stable breeding groups, influenced the genetic structure of the population. These breeding groups were shown to constitute genetically differentiated units with low levels of gene flow between them. The average relatedness among members of a social group was higher than within the population as a whole. As a result of female philopatry coupled with male‐biased natal dispersal, the relatedness of females was higher than that of males, both within social groups and in the whole population. Furthermore, the average relatedness of females within groups was twice the relatedness of females between groups. This study reveals marked fine‐scale, intrapopulation genetic structure, which is attributable to the social behaviour of the European wild rabbit.

Mitochondrial DNA sequence variation and phylogeography of Pimelia darkling beetles on the Island of Tenerife (Canary Islands)

Four morphological taxa of the beetle genus Pimelia (Coleoptera, Tenebrionidae) are known to exist on the Atlantic island of Tenerife. We have obtained DNA sequences for 61 individuals from these taxa across the island for a 200 bp long fragment of the mitochondrial COI gene. In addition, a restriction site polymorphism in the nuclear rRNA ITS-1 sequence was identified and screened in a sample of these individuals using the enzyme Kpn2I. The results were analysed using approaches which allow inferences to be made about the population genetic structure and the mitochondrial genealogy of these closely related beetles. The mtDNA haplotype distribution and the estimates of sequence divergence revealed the presence of two ancient mtDNA lineages which coincide with the disjunct volcanic evolution of the island. The ITS-1 polymorphism was found to be diagnostic of these two lineages. However, the morphological and mitochondrial phylogenies were found to be discordant. We argue that this is possibly the result of rapid morphological change, produced by selection in different habitats, which has been recently superimposed on an older mitochondrial DNA divergence.

Population structure and genetic variation of european wild rabbits (Oryctolagus cuniculus) in east anglia

The European wild rabbit (Oryctolagus cuniculus) is an introduced species in Britain, and populations have been profoundly influenced by both man and disease. In stable environmental conditions, distinct social behaviour is observed, and this social structure leads to significant genetic structuring at the intrapopulation level. In this study, European wild rabbits were sampled from 17 sites across the East Anglian region of Britain and genotyped with nine microsatellite loci. Genotypical proportions deviated significantly from Hardy–Weinberg equilibrium, reflecting a degree of population subdivision and non-random mating. Several estimates of measures of population genetic structure revealed that populations are genetically distinct and have small effective population sizes. These distinctive properties are seen to be the combined effects of the social structure and random drift acting on bottlenecked populations after myxomatosis. It is concluded that the genetic structure seen in rabbit populations today is unlikely to reflect historical structuring present before myxomatosis, but that it results from recent events.

Natal dispersal and genetic structure in a population of the European wild rabbit (Oryctolagus cuniculus)

A combination of behavioural observation, DNA fingerprinting, and allozyme analysis were used to examine natal dispersal in a wild rabbit population. Rabbits lived in territorial, warren based social groups. Over a 6‐year period, significantly more male than female rabbits moved to a new social group before the start of their first breeding season. This pattern of female philopatry and male dispersal was reflected in the genetic structure of the population. DNA fingerprint band‐sharing coefficients were significantly higher for females within the same group than for females between groups, while this was not the case for males. Wrighfs inbreeding coefficients were calculated from fingerprint band‐sharing values and compared to those obtained from allozyme data. There was little correlation between the relative magnitudes of the F‐statistics calculated using the two techniques for comparisons between different social groups. In contrast, two alternative methods for calculating FST from DNA fingerprints gave reasonably concordant values although those based on band‐sharing were consistently lower than those calculated by an ‘allele’ frequency approach. A negative FIS value was obtained from allozyme data. Such excess heterozygosity within social groups is expected even under random mating given the social structure and sex‐biased dispersal but it is argued that the possibility of behavioural avoidance of inbreeding should not be discounted in this species. Estimates of genetic differentiation obtained from allozyme and DNA fingerprint data agreed closely with reported estimates for the yellow‐bellied marmot, a species with a very similar social structure to the European rabbit.

Selection on MHC-linked microsatellite loci in sheep populations

Microsatellites within the major histocompatibility complex (MHC) region have received increasing attention as proxy measures of the level of polymorphism at the Mhc genes themselves. We assessed the diversity of microsatellite loci within or in close proximity of the Mhc genes in several breeds of domestic sheep (Ovis aries) and the wild Mouflon (Ovis orientalis musimon). This was compared to variation at other microsatellite loci scattered throughout the sheep genome. Significantly higher number of alleles were observed at the MHC microsatellites. The sheep breeds studied fell into high- and low-diversity group. This grouping is not related to the agricultural use of the breeds, whether for milk, meat or wool. It is, however, correlated with the geographic origins of the breeds. Southern breeds are genetically more diverse than northern breeds. The observed heterozygosity was in most cases lower than Hardy–Weinberg expectations. The potential impact of selective breeding by man on this is discussed. Neutrality tests indicated that for most of the breeds, the distribution of alleles at the MHC-linked microsatellites are more even than would be expected if the genes were neutral and sampled from populations under drift–mutation equilibrium. Hitchhiking due to tight linkage with alleles at the MHC loci that are under balancing selection is proposed as a possible explanation for this pattern.

Testing for recombination in a short nuclear DNA sequence of the European meadow grasshopper, Chorthippus parallelus

Single‐copy nuclear DNA sequences have high potential as a source of genetic markers for population analyses. However, the difficulties that arise when haplotypes that are the product of recombinational rearrangements are present require additional consideration. Two statistical methods for identifying potential recombinants by detecting anomalies in the distribution of variable sites along sequences were used to screen sequences from a single‐copy nuclear DNA fragment, cpnl‐1, of the European meadow grasshopper (Chorthippus parallelus). Five of the 71 haplotypes in the cpnl‐1 data set showed nonrandom distribution of polymorphic sites using both methods. The second method pinpointed an additional four haplotypes. Estimates of the rate of recombination in the entire data set were obtained using standard methods. It is concluded that cpnl‐1 haplotypes have been involved in recombination or gene conversion events at a rate more than twice the mutation rate. This confirms that recombination and gene conversion are significant factors in the generation of haplotype variation in nuclear gene sequences. The cpnl‐1 haplotypes identified by the tests were present only in populations that have had recent contact; the Balkan and Turkish refugial populations and their postglacial colonies to the north. This is discussed in relation to the phylogenetic inferences drawn from the same data in a previous report.

Are recession populations of the desert locust (Schistocerca gregaria) remnants of past swarms

The desert locust (Schistocerca gregaria) undergoes crowding‐induced phase transformation from solitary to gregarious, which involves changes in behaviour, colour, development, morphometry, fecundity and endocrine physiology. During recession, solitary locusts persist in the central, drier part of the species’ range in small pocket populations that are prone to extinction. During the intermittent upsurges and the subsequent plagues, gregarious swarms attain huge population size and invade a vast area causing major damage to agriculture. A highly variable nuclear DNA marker, a noncoding 3′ end fragment of an antennapedia‐class homeobox gene, was screened in locust samples from Eritrea. Despite the homogenizing potential of plague swarms, the last of which was in 1986–89 and originated in this region, the population genetic structure of solitary phase locusts along the Red Sea coast of Eritrea revealed significant divergence. The pattern of divergence indicated that the invasion of the western and northern plains in the summer of 1995 may not, as reported then, have originated in eastern Chad or western Sudan. A number of interrelated hypotheses have been presented to explain the observed genetic heterogeneity between the sampled populations. We conclude, with caution due to the limited sample sizes, that: (i) geographical isolation between breeding sites during plagues and recession; (ii) the marked differences in the flight behaviour of plague swarms and recession populations; (iii) possible failure of gregarious locusts to solitarize and re‐establish in recession areas; and (iv) the effect of repeated extinction and recolonization in the meta‐population contribute to the maintenance of the genetic structure of recession populations. Potentially productive future research has been identified.