ABRUPT CLINE FOR SEX CHROMOSOMES IN A HYBRID ZONE BETWEEN TWO SPECIES OF MICE.

We compared the patterns of movement of sex chromosomal and autosomal loci along a 160 km transect across a zone of hybridization between M. domesticus and M. musculus in southern Germany and western Austria using seven genetic markers. These included one Y-specific DNA sequence (YB10), two X-specific loci (DXWas68 and DXWas31), and four autosomal isozyme loci (Es-10, Es-1, Mpi-1, and Np-1). Random effects logistic regression analysis enabled us to examine the relationship between M. domesticus allele frequency and geographic distance from the western edge of the hybrid zone and allowed statistical evaluation of differences in cline midpoint and width among loci. More limited movement was observed for all three sex chromosomal markers across the zone compared with three of the four autosomal markers. If differential movement reflects fitness differences of specific alleles (or alleles at closely linked loci) on a hybrid background, then alleles that move to a limited extent across a hybrid zone may contribute to hybrid breakdown between two species. The limited flow of both X- and Y-specific alleles suggest that sex chromosomes have played an important role in Mus speciation.

MITOCHONDRIAL DNA DIVERSITY IN THE SEA URCHINS STRONGYLOCENTROTUS PURPURATUS AND S. DROEBACHIENSIS.

Restriction-fragment analysis was used to measure mitochondrial DNA (mtDNA) variability in 79 individuals of two species of temperate sea urchins. For the purple urchin Strongylocentrotus purpuratus, individuals were collected 1,500 km apart in 1985 and again from the same localities in 1988 (about one urchin generation). Twenty mtDNA genotypes belonging to four clades were found among 38 individuals. All four clades were found in both localities and in both years. Genetic structure was further tested by calculating the degree of interdeme genetic variation (GST ) and comparing this value to the GST 's from randomly shuffled data. No geographic structure was found. For S. droebachiensis, only six mtDNA genotypes were found among 41 individuals collected from the Pacific and Atlantic oceans. More than 80% of the individuals belonged to two genotypes. The genotype that dominated collections in the Pacific also occurred in the Atlantic; however, a common Atlantic genotype was never found in the Pacific. These two genotypes were identical at 64 of 65 restriction sites, and were only 0.2% divergent from each other. GST analysis confirmed that there were significant genetic differences between Atlantic and Pacific populations. The small divergence between genotypes suggests recent, but not continuous, migration. These marine species show smaller genotypic differences than terrestrial species over similar spatial and temporal scales. Both recruitment of adults from planktonic larval pools and the spread of sibling larvae over large distances from parents probably act as buffers to genetic differences in species with planktonic life-history phases.

EVOLUTIONARY INFERENCES FROM RESTRICTION MAPS OF MITOCHONDRIAL DNA FROM NINE TAXA OF XENOPUS FROGS.

Restriction endonuclease cleavage maps were prepared by the double digestion method for mitochondrial DNAs (mtDNAs) purified from Xenopus borealis, X. clivii, X. fraseri, X. muelleri, X. ruwenzoriensis, X. vestitus, X. laevis victorianus, X. l. laevis, and a variant of X. laevis designated X. laevis "davis." An average of 21 cleavage sites per genome were mapped with 11 restriction endonucleases. Among the four invariant sites found are three conserved not only among the Xenopus mtDNAs tested but also among nearly all vertebrate mtDNAs examined to date. Two of these are Sac II sites in the 12S and 16S ribosomal RNA genes, and one is a Hpa I site in the gene for asparagine transfer RNA. These three sites permit the alignment and comparison of mtDNAs from different vertebrate classes. Although most of the differences observed among the Xenopus maps are attributable to point mutations causing gain or loss of restriction sites, the maps also differ by three large length mutations in or near the displacement loop. Phylogenetic analysis of 30 informative sites suggests that those members of the laevis species-group that have 36 chromosomes per somatic cell can be divided into three subgroups: 1) X. borealis, X. clivii, and perhaps X. fraseri (the "borealis" subgroup), 2) X. muelleri, and 3) the subspecies of X, laevis. The mtDNA of the hexaploid (2n = 108) species, X. ruwenzoriensis, is most similar to that of taxa in the latter two subgroups, which contrasts with the morphological similarity of this species to X. fraseri. X. ruwenzoriensis may be an allopolyploid with a mother (the contributor of the cytoplasmic mtDNA genome) on the X. laevis or X. muelleri lineage and a father on the X. fraseri lineage. We present a model showing how mtDNA and nuclear genomes can yield contrasting phytogenies for species-groups that have undergone several rounds of interspecific hybridization. Comparison of mitochondrial and nuclear sequence divergences suggests that Xenopus mtDNA, like that of mammals and birds, evolves faster than nuclear DNA. Genetic distances among mtDNAs of Xenopus species are very large, generally approaching or exceeding one substitution per nucleotide.