Genetics of speciation in the Aedes (Stegomyia) scutellaris group (Diptera: Culicidae) V. Chromosomal relationships among five species

Genome evolution in the mosquitoes and other closely related members of superfamily Culicoidea

Karyotypes and nuclear DNA amounts of 36 species belonging to twelve genera of mosquitoes and other closely related taxa in the superfamily Culicoidea (Dixidae, Chaoboridae, and Culicidae) were studied. The results showed an eight-fold range in haploid DNA amounts and a 4.5-fold range in total chromosomal lengths. It is concluded that the evolutionary trend in this superfamily has been an overall increase in total haploid DNA amounts from the primitive Dixidae to the highly evolved Culcidae, with a concomitant increase in overall chromosome sizes.

Mosquito genomes: structure, organization, and evolution

A great deal of information has been accumulated on chromosome numbers and heterochromatin distribution as well as on genome size and organization in the mosquito family Culicidae. A number of trends in genome evolution emerge when these data are reviewed in light of recent cladistic phylogenies of Culicidae and its sister families. Anophelinae have heteromorphic sex chromosomes and a small genome size, and repetitive elements are distributed in a long-period interspersion pattern. In contrast, Culicinae have homomorphic sex chromosomes, and repetitive DNA is organized in a short-period interspersion pattern. There has been a general increase in genome size during the evolution of culicine tribes. The organization of the ancestral culicid genome remains uncertain awaiting studies on genome organization in Chaoboridae-Corethrellidae taxa. The most parsimonious hypothesis for the evolution of sex chromosomes and genome organization in Culicidae would be that homomorphic sex chromosomes and a long-period interspersion pattern was ancestral in lineages leading to Toxorhynchitinae and Culcinae. Larger genomes developed in subsequent culicine lineages through accumulation of short-period interspersed repetitive elements. Heteromorphic sex chromosomes evolved early in the evolution of Anophelinae, and a long-period interspersion pattern was retained. The alternative scenario proposed by Rao and Rai (1987a) is that Culicidae arose from a chaoborid Mochlonyx-like ancestor with heteromorphic sex chromosomes and possibly short-period interspersion. This scenario would require the loss of heteromorphic sex chromosomes in the lineage leading to Toxorhynchitinae and Culicinae and the "shedding" of repetitive elements in the lineage leading to Anophelinae. Several interesting patterns have emerged from studies of C-banding, and the distribution of heterochromatin in Culicidae and phylogenies derived from these studies are supported by the modern cladistic analyses. Recent intensive multipoint linkage map studies suggest that recombination frequencies per genome have remained relatively constant over the course of culicid evolution such that Anophelinae, with a relatively small genome size, has a linkage map of similar size to Aedini. As a consequence, taxa in Anophelinae have higher amounts of recombination per haploid genome size than Culicinae. Although several key questions have yet to be addressed, the Culicidae remain one of the best-studied systems of genome evolution in animals.

Inter and intraspecific variation in nuclear DNA content in Aedes mosquitoes

Haploid nuclear DNA of 23 species of Aedes, as determined by Feulgen cytophotometry, was found to vary 3-fold. This was accompanied by a 2-fold variation in total chromosomal length. There was a significant correlation (r = 0.765, P less than 0.001) between these two parameters. Genome size varied from 0.87 pg to 1.3 pg among 10 strains of Aedes albopictus, from wide geographic regions. Large scale differences in chromosomal DNA amounts have accompanied speciation and evolution in aedine mosquitoes.