Flow of mitochondrial DNA across a species boundary.

Abstract

Restriction analysis shows that wild Scandinavian mice belonging to the species Mus mtuculus contain the mitochondrial DNA of a neighboring species, M. domesticus. This demonstration results from comparisons of Scandinavian mice with authentic M. dwometicus and M. muscdus from other parts of Europe. Electrophoretic and immunological analysis of eight diagnostic proteins confirms that mice from north of the hybrid zone in Denmark are M. musculus in regard to their nuclear genes. In contrast, the mice tested from this region and a nearby part of Sweden have exclusively M. domesticus types of mitochondrial DNA. Phylogenetic analysis of the restriction maps suggests that the mitochondrial DNAs found in Scandinavian M. musculus could stem from a single M. domesticus female.The growing use of mtDNA as a tool for genetic research on animal populations (1, 2) makes it important to compare the ability of nuclear and mitochondrial genomes to move between populations. [mtDNA differs conspicuously from nuclear DNA not only by being outside the nucleus but also by existing in thousands of copies per cell, being inherited maternally, and evolving quickly (3,4).] Such a comparison can be made by examining the distribution of genes across a hybrid zone-i.e., a geographic zone where two species meet and interbreed but where there is limited flow of nuclear genes (5).Of all the hybrid zones examined by both organismal and molecular biologists, that between two species of mice in Denmark is the best known (6-8). The comprehensive study by Hunt and Selander (7) of proteins encoded by the nuclei of 2,696 mice caught at 44 Danish localities delineated the hybrid zone as regards nuclear genes. In addition, the protein evidence agrees with anatomical evidence as to the geographic location of this hybrid zone (6-8).Further protein work has shown how these Danish mice are related to other commensal mice (9, 10). Commensal mice are those species that live in close association with buildings used by humans. They contrast with aboriginal mice (in Europe: Mus spretus, M. hortulanus, and M. abbotti), which live predominantly independent of human dwellings and, in nature, do not interbreed with commensal mice (10-12). According to a phylogenetic analysis of the protein data, there are two commensal mouse species in Europe. One, known as M. domesticus, lives in southern Denmark, in most of the rest of western Europe, and around the Mediterranean Sea (11, 12) (see Fig. 1). The second, M. musculus, lives in northern Denmark, the rest of Scandinavia, and eastern Europe (11, 12). The hybrid zone defined by Ursin (6) and Selander and co-workers (7, 8) is the meeting place of M. domesticus and M. musculus in Denmark (see Fig. 1). These two types of mice are sometimes considered as semispecies. Our decision to refer to them as separate species is based not only on extensive morphological and biochemical evidence (10-12) but also on the observation that there is a high incidence of sterility in the male offspring of crosses between M. musc...

Mitochondrial DNA variability in natural populations of Hawaiian Drosophila. I. Methods and levels of variability in D. silvestris and D. heteroneura populations

“…Because of the recent divergence of these two species (more recent than the age of the island of Hawaii which is about 500,000 years [McDougall and Swanson, 1972]) convergence of restriction site state is unlikely to explain the high genetic similarity among these individuals (see Templeton, 1983 a). Hybridisation between the two species (Ferris et a!., 1983;Powell, 1983) and the effects of stochastic branching processes on maternal lineage survivorship (Avise et a!., 1983;Avise et al, 1984) as discussed in the next section are two possible explanations for this observation.…”

“…If hy6ridisation has occurred between D. silvestris and D. heteroneura, its impact on the genetic structure of the species involved is not the same as in other animals where hybridisation has been inferred from mtDNA analysis (Ferris et aL, 1983;Powell, 1982) and in which there are distinct differences between the nuclear genomes of the species involved. In these cases mtDNA gene poois of hybridizing species become homogenized, with loss of one of the mtDNA lineages.…”

“…Genetic distance between this line and D. heteroneura mtDNA restriction endonuclease data has recently been applied to the population genetics of humans and apes (Brown, 1980;Cann et a!., 1982;Ferris et al, 1981), lizards (Brown and Wright, 1979), rodents (Lansman et a!., 1981;Brown and Simpson, 1981;Ferris et al, 1983;Avise et al, 1983) and Drosophila (Shah and Langley, 1979;Powell, 1983;Powell and Zuniga, 1983). Vertebrate mtDNA is relatively easy to analyse as it is available in relatively large amounts per individual (Giles et aL, 1980;Lansman et a!., 1981;Ferris et al, 1983). As Powell (1983) points out, however, mtDNA restriction techniques in Drosophila systems usually demands that isofemale lines are used to obtain enough tissue to isolate mtDNA.…”

“…Analysis of mitochondrial DNA (mtDNA) using restriction endonuclease digestion is useful in the study of evolution in matriarchal lineages (Brown and Wright, 1977;Lansman et al, 1981;Avise et aL, 1979;Ferris et a!., 1983). The rapid evolution of mtDNA (Brown eta!., 1979;Brown, 1980;Avise et a!., 1979) means that mtDNA restriction site analysis is also valuable in studying closely related taxa.…”

On the subspecific origin of the laboratory mouse

“…Interestingly, the M. m. molossinus strain MOLF carries a domesticus Y chromosome. Flow of mitochondrial DNA across subspecies boundaries 37 and discordant phylogenetic patterns between mitochondria and Y chromosome have been reported in wild population 19 , indicating that secondary introgression after radiation of the subspecies 20 might have contributed to the pattern of intersubspecific introgression observed in the wild-derived inbred strains, in addition to accidental "contamination" in the laboratory.…”