Insights from mosquito evolution: Patterns, tempo and speciation

Abstract

A great deal of information has accumulated on chromosome morphology and evolution, heterochromatin distribution and differentiation, and molecular structure, organization and evolution of genomes in the mosquito family Culicidae. Whereas numerically the haploid chromosome number (n = 3) has remained virtually unchanged, extensive variation exists at different levels of genomic structure and organization. A number of trends in genome evolution emerge when these data are considered in light of cladistic phylogenies of Culicidae and its sister families. Anophelinae have heteromorphic sex chromosomes, 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 taxa. The most likely hypothesis for the evolution of sex chromosomes and genome organization in Culicidae would be that homomorphic sex chromosomes and a long-period interspersion pattern are ancestral in lineages leading to Toxorhynchitinae and Culicinae. 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. An alternative route may be that Culicidae arose from a Chaoborid Mochlonyx-like ancestor with heteromorphic sex chromosomes and possibly short-period interspersion. This would require the loss of heteromorphic sex chromosomes in the lineage leading to Toxorhynchitinae and Culicinae and ‘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.