Introgression of extranuclear genomes in finite populations: nucleo-cytoplasmic incompatibility

LOCAL CYTONUCLEAR EXTINCTION OF THE GOLDEN-WINGED WARBLER

I compared the mtDNA compositions of two adjacent populations of Vermivora chrysoptera (golden-winged warbler) at different stages of transient hybridization with its sister species V. pinus (blue-winged warbler). Pinus mtDNA introgresses asymmetrically and perhaps rapidly into chrysoptera phenotypes without comparable reverse introgression of chrysoptera mtDNA into replacing pinus populations. Pinus mtDNA was virtually fixed (98%) in an actively hybridizing lowland population with varied phenotypes. Pinus mtDNA increased from 27% (n = 11) in 1988 to 70% (n = 10) in 1992 in successive samples of a highland population in the initial stages of hybridization. This population comprised mostly pure and slightly introgressed chrysoptera phenotypes. The rapid pace of asymmetrical introgression may be the result of initial invasion of chrysoptera populations by pioneering female pinus and/or an unknown competitive advantage of pinus females and their daughters over chrysoptera females.

ASSOCIATIONS BETWEEN MITOCHONDRIAL AND NUCLEAR GENOTYPES IN CUTTHROAT TROUT HYBRID SWARMS

We examined mtDNA and nuclear allozyme genotypes in hybrid populations formed from interbreeding of westslope cutthroat trout (Oncorhynchus clarki lewisi) and Yellowstone cutthroat trout (O. c. bouvieri). These subspecies show substantial genetic divergence (Nei's D = 0.30; mtDNA P = 0.02). Diagnostic alleles at multiple nuclear loci and two distinct mtDNA haplotypes segregate in the hybrids. Nuclear and mtDNA genotypes are largely randomly associated, although there is slight disequilibrium in both nuclear and cytonuclear measures in some samples. Consistent positive gametic disequilibria for three pairs of nuclear loci confirm one previously reported linkage, and indicate two more. Allele frequencies provide no evidence for selection on individual chromosome segments. However, westslope mtDNA haplotype frequencies exceed westslope nuclear allele frequencies in all samples. This may be explained by differences in the frequency of occurrence of reciprocal F₁ matings, by viability, fertility, or sex ratio differences in the progeny of reciprocal matings, or by weak selection on mtDNA haplotypes.

Dubious maternal inheritance of mitochondrial DNA in D. simulans and evolution of D. mauritiana

Within-line heterogeneity has been found in the mitochondrial DNA (mtDNA) in two isofemale lines of D. simulans. The co-existing types, S and M, were typical of the mtDNA in D. simulans and in D. mauritiana, respectively, their nucleotide divergence per site being ca. 2·1%. Segregation analysis confirmed that some individuals in these lines were heteroplasmic and suggested incomplete maternal inheritance of mtDNA in Drosophila. Examination of other lines of D. simulans revealed that the M type of D. mauritiana occurs at 71% in Réunion, 38% in Madagascar and 0% in Kenya. This finding and interspecific sequence comparisons of both M types indicate that D. mauritiana diverged from D. simulans probably less than 240000 years ago.

Low genetic divergence obscures phylogeny among populations of Sphenodon, remnant of an ancient reptile lineage

Tuatara (two species of Sphenodon) are the last representatives of a branch of an ancient reptilian lineage, Sphenodontia, that have been isolated on the New Zealand landmass for 82 million years. We present analyses of geographic variation in allozymes, mitochondrial DNA, nuclear DNA sequences, and one-way albumin immunological comparisons. These all confirm a surprisingly low level of genetic diversity within Sphenodon for such an ancient lineage. We hypothesise a recent extended population bottleneck, probably during the Pliocene/Pleistocene glaciation cycles, to explain the current paucity of variation. All data sets reveal clear genetic differentiation between the northern populations and those in Cook Strait, but offer conflicting views of the history and taxonomic relationships of the Cook Strait population on North Brother Island, currently recognised as Sphenodon guntheri. Allozymes show this population to be the most divergent of all tuatara populations, but preliminary mitochondrial DNA data indicate few differences between S. guntheri and Cook Strait Sphenodon punctatus. Interpretation of the trees is confounded by the lack of a suitable outgroup. As in other cases of conflicting nuclear and mitochondrial data sets, the different data sets likely reveal different aspects of the animals' evolutionary history, and introgression is not uncommon between species pairs.

A MODEL FOR CHLOROPLAST CAPTURE

Chloroplast capture, the introgression of a chloroplast from one species into another, has been frequently suggested as the explanation for inconsistencies between gene trees based on nuclear and cytoplasmic markers in plants. We use a genetic model to determine the conditions for capture to occur, and we find that they are somewhat more general than those given in earlier verbal arguments. Chloroplast capture can occur if cytoplasm substitution provides an advantage in seed production. This can happen through reallocation to the female function when cytonuclear incompatibilities cause partial male sterility, but also under more general conditions. Capture is promoted by nuclear incompatibilities between the two genomes (or a low heterosis in F1 hybrids) and by partial selfing when hybridization causes a decrease in the selfing rate and inbreeding depression is strong. We discuss empirical predictions that can be used to test this mechanism.

Transfer of mitochondrial DNA from the northern red-backed vole (Clethrionomys rutilus) to the bank vole (C. glareolus)

Using a silver staining method to detect DNA fragments produced by restriction enzymes, it was possible to compare mitochondrial DNAs (mtDNAs) from 85 individuals of the bank vole (Clethrionomys glareolus) trapped at 25 localities in Fennoscandia. There are two distinctly different mtDNA lineages, one occurring in southern and central Fennoscandia and the other in the northern parts. A fragment comparison method shows about 12.7% nucleotide sequence divergence between these two lineages. This major difference between animals of the same species could theoretically be explained by intraspecific lineage survivorship independent of species hybridization, or by introduction of an atypical mtDNA via hybridization with a closely related species. Analysis of mtDNAs from the two other Clethrionomys species present in Fennoscandia (C. rutilus and C. rufocanus) shows that the mtDNA of northern C. glareolus is very similar to that of C. rutilus and that the mtDNA lineages of these two species cluster together in a phenogram, with small genetic distances among them. By contrast, electrophoresis of proteins encoded by 17 nuclear loci reveals fixed allelic differences between these two species at 8 loci. Hence the presence of two distinctly different mtDNA lineages within C. glareolus may be a consequence of a limited episode of hybridization between C. glareolus and C. rutilus, probably during the postglacial recolonization of Fennoscandia 8000-13,000 years ago.

Interspecific mitochondrial DNA transfer and the colonization of Scandinavia by mice

Restriction enzymes were used to search for genetic variability at 162 cleavage sites in mitochondrial DNA (mtDNA) purified from 22 mice caught at seven Swedish localities. Although all of these mice bear the nuclear genes ofMus musculus, they bear the mtDNA ofM. domesticusexclusively. Yet, some of the Swedish localities are 750 km away from the hybrid zone between these two species. Furthermore, only one type of mtDNA was found at the seven Swedish localities; this type was found before at an eighth locality in Sweden as well as in Jutland north of the hybrid zone. The apparent lack of mtDNA divergence in the mouse population of Sweden contrasts with the extensive divergence usually found within other geographic areas in Europe, Africa and North America. Electrophoretic analysis of proteins encoded by nuclear genes indicates that the Swedish mice have lower average heterozygosity than Danish and Central European populations ofmusculusmice. These findings lead us to suggest that the source of the commensal mouse population in Sweden was a small propagule that originated from a population situated only a few kilometres to the east of the point at which the hybrid zone on the European mainland meets the Baltic Sea, namely on East Holstein. Such a founder event may have been associated with the spread of farming from north Germany into Sweden about 4000 years ago.