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We explore the mutational processes giving rise to microsatellite diversity by analysing allele lengths in 6045 human microsatellite markers drawn from the CEPH panel. Assuming a general mutation-drift process generating this diversity, the bias of the mutation distribution cannot be directly estimated from such data. However, inferences can still be made about the degree and sign of the asymmetry of the mutation distribution. We consider statistics based on moments of the observed length distribution, and derive their relevant analytical properties, showing that they have a high sampling variance. We conclude that moment estimators applied to allele length frequencies within the CEPH database could not be used to reject a null hypothesis of no bias even if bias was present. However, an order parameter does provide evidence of asymmetrically biased mutation: there is an unambiguous preponderance of alleles in which the shortest locus is the most frequent. It will be important to further characterize the sampling properties of such order parameters to estimate the magnitude of any mutation bias and the sensitivity of this estimation to the mutation model assumed.

Over the last three decades, techniques have become available to screen large numbers of individual animals for genetic variability at the molecular level. Earlier work employed gel electrophoresis to resolve slight changes in the amino acid sequence of proteins. More recently, polymorphism at the level of the DNA itself has become amenable to screening through the discovery of a class of enzymes, restriction endonucleases, which cleave DNA at specific sites. By focusing on specific classes of DNA sequence, each with its own characteristic rate and mode of evolution, different degrees of genetic resolution may be obtained. At the fastest end of the spectrum, the extremely high rates of gain and loss of repeats from minisatellite arrays forms the basis of the technique of DNA fingerprinting, capable of uniquely identifying individuals and assignment of parentage. At the population level, internal mechanisms of molecular turnover result in the homogenisation of repeated sequences and can result in population-characteristic markers. The small extranuclear mitochondrial genome is inherited only through the maternal line and thus provides an informative tool for investigating haplotype lineages. All these techniques, and others, are discussed in relation to how they may be used to determine the basic population parameters which may be important in a critical assessment of an endangered species: the effective population size, population substructure and various aspects of the generation and effects of having low genetic variability. In some instances several techniques may be equally suitable, in others only one will do. We therefore discuss other factors which may influence the choice of methodology, such as approaches for tissue sampling, sample preservation and cost. Amongst these, the technique of PCR (Polymerase chain reaction), a means by which minute quantities of a specific DNA sequence may be enzymatically amplified, can be singled out as one which will influence profoundly the scope and power of future molecular genetic analysis. sample preservation and cost. Amongst these, the technique of PCR (Polymerase chain reaction), a means by which minute quantities of a specific DNA sequence may be enzymatically amplified, can be singled out as one which will influence profoundly the scope and power of future molecular genetic analysis.

Microsatellites are stretches of repetitive DNA, where individual repeat units comprise one to six bases. These sequences are often highly polymorphic with respect to repeat number and include trinucleotide repeats, which are abnormally expanded in a number of diseases. It has been widely assumed that microsatellite loci are as likely to gain and lose repeats when they mutate. In this review, we present population genetic and empirical data arguing that microsatellites, including normal alleles at trinucleotide repeat disease loci, are more likely to expand in length when they mutate. In addition, our experiments suggest that the rates of expansion of such sequences differ in related species.

Mitochondrial DNA haplotypes of humpback whales show strong segregation between oceanic populations and between feeding grounds within oceans, but this highly structured pattern does not exclude the possibility of extensive nuclear gene flow. Here we present allele frequency data for four microsatellite loci typed across samples from four major oceanic regions: the North Atlantic (two mitochondrially distinct populations), the North Pacific, and two widely separated Antarctic regions, East Australia and the Antarctic Peninsula. Allelic diversity is a little greater in the two Antarctic samples, probably indicating historically greater population sizes. Population subdivision was examined using a wide range of measures, including F-st, various alternative forms of Slatkin's R(st), Goldstein and colleagues' Delta mu, and a Monte Carlo approximation to Fisher's exact test. The exact test revealed significant heterogeneity in all but one of the pairwise comparisons between geographically adjacent populations, including the comparison between the two North Atlantic populations, suggesting that, gene flow between oceans is minimal and that dispersal patterns may sometimes be restricted even in the absence of obvious barriers, such as land masses, warm water belts, and antitropical migration behavior. The only comparison where heterogeneity was not detected was the one between the two Antarctic population samples. It is unclear whether failure to find a difference here reflects gene flow between the regions or merely lack of statistical power arising from the small size of the Antarctic Peninsula sample. Our comparison between measures of population subdivision revealed major discrepancies between methods, with little agreement about which populations were most and least separated. We suggest that unbiased R(st) (UR(st), see Goodman 1995) is currently the most reliable statistic, probably because, unlike the other methods, it allows for unequal sample sizes. However, in view of the fact that these alternative measures often contradict one another, we urge caution in the use of microsatellite data to quantify genetic distance.

Colonially breeding gray seals are polygynous. Males are larger than females, compete with each other for position among aggregated females, and contribute no parental care. Genetic analysis of pups born on the island of North Rona, Scotland, reveals large numbers of full siblings, although dominant males father disproportionately few of these. This result cannot be explained by mating patterns based solely on male dominance and the spatio-temporal organization of the breeding colony. Instead, many full siblings must result from choices favoring previous parental combinations. Thus, polygyny and partner fidelity appear to operate simultaneously in this breeding colony.

Long-finned pilot whales swim in large, extremely cohesive social groups known as pods. Molecular typing revealed that pod members form a single extended family. Mature males neither disperse from nor mate within their natal pods, a situation unusual for mammals. Such behavior could be explained in terms of inclusive fitness benefits gained by adult males helping the large number of female relatives with which they swim.

DNA fingerprinting of non-human populations is beginning to fulfill its early promise, and in the past year there has been a flush of papers on mammalian breeding systems. However, many people, particularly field workers, believe that progress in this area has been slow. Attention is now focused on two amenable alternatives: microsatellite polymorphisms and randomly amplified polymorphic DNA. Of these, it is probable that microsatellites hold the key to rapid, efficient and highly informative screening of the genetic variability that exists within natural populations.

Length polymorphisms within simple-sequence loci occur ubiquitously in non-coding eukaryotic DNA and can be highly informative in the analysis of natural populations. Simple-sequence length polymorphisms (SSLP) in the long-finned pilot whale Globicephala melas (Delphinidae) have provided useful information on the mating system as well as on the genetic structure of populations. We have therefore tested whether the polymerase chain reaction primers designed for Globicephala could also be used to uncover variability in other whale species. Homologous loci could indeed be amplified from a diverse range of whales, including all toothed (Odontoceti) and baleen whales (Mysticeti) tested. Cloning and sequencing these loci from 11 different species revealed an unusually high conservation of sequences flanking the simple-sequence stretches, averaging 3.2% difference over 35-40 Myr. This represents the lowest divergence rate for neutral nucleotide positions found for any species group so far and raises the possible need for a re-evaluation of the age of the modern whales. On the other hand, the high conservation of non-coding sequences in whales simplifies the application of SSLP DNA fingerprinting in cetacean species, as primers designed for one species will often uncover variability in other species.

They've done it again! Those friendly people who brought you DNA fingerprinting have now revealed genetic variation on an extraordinarily fine scale. Not only that: in a recent paper describing DNA sequence differences between alleles at a minisatellite (fingerprint) locus, Alec Jeffreys, Rita Neumann and Victoria Wilson of Leicester University have also answered fundamental questions about the rate and mode of generation of this variation I. DNA fingerprints-',.*, those unevenly spaced ladders of bands, which in many species are unique for an individual, are the result of screening several extremely variable loci simultaneously. Each band represents an allele at a particular locus and is thus one of a pair. Its partner is usually elsewhere in the fingerprint profile (heterozygous state) but may occasionally occur at the same point (homozygous state). The DNA sequences detected by fingerprint probes are very short (e.g. 16 base pairs for the Jeffreys probe X33.15) and often occur in strings, known as tandem repeat arrays or minisatellite DNA. Variation in allele position on a fingerprint profile is due to differences in the length of the DNA fragments detected, which in turn result from variation ,7:~ the number of repeats within an array. At minisatellite loci, repeat units appear to be easily gained or lost and different alleles can have anything from a handful to hundreds of repeats. Although DNA fingerprints are useful for de termining close kin, for many other purposes (e.g. population surveys) it is useful to have locus-specific information. Unfortunately, sorting out which bands