The interdependence of B chromosomes, nucleolar organizer expression, and larval development in the blackfly species Cnephia dacotensis and Cnephia ornithophilia (Diptera: Simuliidae)

Abstract

A primary relationship is found between nucleolar organizer (NO) expression (puffing) of the A complement and numbers of B chromosomes in Cnephia dacotensis, such that a high incidence of B's is associated with nonexpression of the NO independent of sampling time and larval size. NO expression can be categorized into three zygote classes: + +, + −, and − −, where + indicates puffing of one constituent and − indicates little or no puffing. The + + types generally mature earlier than the + − and − − types. These observations suggest that development rates are a function of NO expression: − − types do not mature at all unless they have B's and (or) secondary nucleoli. Population comparisons confirm the prediction that as the frequency of individuals with one or more B's decreases, the frequency of individuals with expressed constituents increases. A sex bias in NO expression exists with more expression in females. Additional plasticity is conferred to the system through an apparent conversion phenomenon which reduces X⁺X⁺ expression in females and dosage compensates X⁻Y⁻ males. Both of these effects are B-chromosome dependent and are lost in zero B populations. It is proposed that B–A NO interactions constitute a system for more efficient rRNA production by optimizing synthesis between the sexes. This system may account for the wider range and occupation of more diversified habitats of C. dacotensis than for other members of the group. A similar but less dynamic role is proposed for the effects of B chromosomes in C. ornithophilia.