Evolution of mitochondrial DNA in the brook stickleback, Culaea inconstans (Teleostei: Gasterosteidae): Description and transmission genetics of sequence and length variation.

Abstract

Most animal mtDNAs are small, 15 to 18 kilobases (kb) in size. Unusual variation in mtDNA size occurs in the brook stickleback. The gene content and structure of the size variation is investigated using restriction mapping and transfer-hybridization techniques. The evolution of the size variation is studied by restriction mapping and phylogenetic analysis of 91 restriction sites occurring in Culaea inconstans, Gasterosteus aculeatus, and Pungitius pungitius. Transmission genetics of mtDNA and the effects of mutation, selection, and paternal inheritance are examined in single-generation crosses of females carrying two or more mtDNA size classes. Brook stickleback mitochondrial genome size is estimated to be 17.4-24 kb and often varies within individuals (heteroplasmy). An additional one-kb sequence and small size variation (100-300 basepairs, bp) occurs in some fish. Large (2.7-5.8 kb) tandem duplications encompassing the control region and adjacent structural genes are seen in many individuals. Duplications are of two types, N and S; these differ in length, gene content, and frequency. Duplication N is 3.2-5.4 kb. Endpoints are stable; size variation is due to a tandemly repeated segment about 270 bp long. Duplication S is 2.7-5.8 kb, with variable endpoints. MtDNAs group in two major lineages, A and B, which differed by 6.4% (s.e. = 1.2%) in nucleotide sequence. Non-overlapping distributions of the A and B lineages suggest an historic division within the species. New Mexico and Michigan stickleback mtDNAs are nearly identical, suggesting that the former populations may not be a Pleistocene relict. Duplication N occurs in all A genomes (n = 121). In contrast, duplication S occurred in only 117 of 174 fish from lineage B. Phylogenetic analysis and the incorporation of sequence modifications into duplications argue for multiple origins of N and S within Culaea. Specific aspects of animal mtDNA inheritance are poorly known. An estimate of mtDNA population size in organismal generations, $N\sb{g},$ is defined, and a variance-sampling model is extended to multiple alleles. Males do not significantly contribute to mtDNA diversity in Culaea. Mutation and size selection (for larger and smaller genomes) influence mtDNA allele frequencies. MtDNAs are subject to strong drift, induced by random segregation of mtDNA into germ cells throughout early embryogenesis.