Coyote Genes in Declining Wolves
The Biology of Mitochondria and Chloroplasts
Mitochondrion and Chloroplast Structure
The Genetics of Organelle-Encoded Traits
The Endosymbiotic Theory
Gene Structure and Organization of mtDNA
Nonuniversal Codons in mtDNA
Replication, Transcription, and Translation of mtDNA
Evolution of mtDNA Chloroplast DNA
Gene Structure and Organization of cpDNA
Replication, Transcription, and Translation of cpDNA Evolution of cpDNA
The Intergenomic Exchange of Genetic Information
Mitochondrial DNA and Aging in Humans
North America is home to two wild canids: the gray wolf (Canis lupus) and the coyote (Canis latrans). Before European settlement, gray wolves ranged across much of North America, occupying forest, plains, desert, and tundra habitat (IFigure 20.1). Coyotes had a more limited distribution, being confined primarily to plains and deserts. With the expansion of European settlement and the development of intensive agriculture in the eighteenth and nineteenth centuries, the distribution of wolves and coyotes changed dramatically (see Figure 20.1). Wolf populations in North America declined precipitously owing to habitat destruction and deliberate extermination. In contrast, coyote populations expanded, probably because competition from wolves was eliminated and because coyotes were better able to adapt to human disruption of the ecosystem. Today coyotes are found throughout most of North America.
Habitat alternation and changes in their distributions have increased interactions between wolves and coyotes in recent times, providing more opportunities for hybridization between the two species. In captivity, wolves and coyotes will interbreed and produce fertile hybrids. Large coyotes in New England and southeastern Canada may be the result of hybridization between wolves and coyotes in these areas. To what extent is hybridization occurring between coyotes and wolves in nature?
To answer this question, Niles Lehman and his colleagues studied DNA in the mitochondria of wolves and coyotes. Mitochondrial DNA (mtDNA) can be helpful in determining hybridization between animals for two reasons: (1) in animals, it is inherited only from the female
20.1 Original and current ranges of the gray wolf (Canis lupus) and the coyote (Canis latrans). Originally, the gray wolf occupied most of North America, but its current range is restricted to northern Minnesota, Canada, and Alaska. The coyote originally occupied the plains and desert habitat in the midwestern United States and Mexico. Today, the coyote is found throughout most of North America. The results of studies of mitochondrial DNA reveal that hybridization is occurring between wolves and coyotes.
parent and (2) it evolves rapidly. Lehman and his colleagues gathered tissue and blood samples from more than 500 gray wolves and coyotes in North America, extracted mtDNA from the samples, and analyzed restriction fragment length polymorphisms (see Chapter 18) in the mtDNA. The results of his study revealed two major clusters of mtDNA among the canids: one consisting of wolf mtDNA and another of coyote-like mtDNA. Surprisingly, the coyote-like mtDNA cluster included several samples that had been obtained from wolves, indicating that some wolves possessed coyotelike mtDNA. The wolves with coyote-like mtDNA were all from the U.S.-Canadian border area, which has recently been invaded by coyotes. No wolves from Alaska or northern Canada possessed coyote-like mtDNA. All the coyotes had only coyote-like mtDNA.
These results indicate that unidirectional hybridization has taken place between coyotes and wolves: coyote mtDNA has entered wolf populations, but wolf mtDNA has not entered coyote populations. The fact that in animals mtDNA is inherited only from the female parent implies that female coyotes are mating successfully with male wolves and the wolf-coyote hybrids are backcrossing with wolves, introducing coyote genes into wolf populations.
These findings have important implications for the future of gray wolves in North America. Hybridization between wolves and coyotes threatens to erode the genetic integrity of wolves. As human activities encroach on areas occupied by wolves, wolves and coyotes will be forced into closer contact and there will be more hybridization; the wolf genome (both mitochondrial and nuclear) will become increasingly diluted by coyote DNA. Current efforts to reintroduce wolves into former territories, which are now occupied by coyotes, may lead to further hybridization, ultimately harming, rather than helping, wolf populations.
DNA sequences found in mitochondria and other organelles possess unique properties that make these sequences useful in the fields of conservation biology, evolution, and genetic diseases. Uniparental inheritance exhibited by genes found in mitochondria and chloroplasts was discussed in Chapter 5; the present chapter examines molecular aspects of organelle DNA. We begin by briefly considering the structures of mitochondria and chloroplasts, the inheritance of traits encoded by their genes, and their evolutionary origin. We then examine the general characteristics of mtDNA, followed by a discussion of the organization and function of different types of mitochondrial genomes.
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