Sex ratio distortion in populations and its possible role in insect suppression : Experimental studies with strains of Drosophila melanogaster carrying cytoplasmically-inherited male-killing spiroplasmas.

Abstract

The possible role of cytoplasmic sex ratio (SR) distortion in insect population control has been assessed experimentally. In Drosophila melanogaster the agent responsible for SR distortion is a male-killing spiroplasma and SR strains are characterized by females producing daughters but few, if any, sons. Populations were constructed of wildtype and SR females (of various male-kill effectivenesses) plus males, in different proportions. The frequency of females in the populations was monitored regularly. Under some circumstances the frequency of females increased but not to fixation or population collapse. In most instances the frequency of females declined (to as low as 25% in some cases), then went through cycles of increase and decrease, with gradually reduced amplitude over time. This pattern of fluctuations was repeatable over cages.Independently, it was shown that SR females persisted in the cages for the length of the experiments. The unexpected extremely low frequencies of females found in cages established with an excess of females, raised the possibility of distorting a population by SR releases, and then when the female frequency is at its lowest point to add sterile male or similar load inducers to suppress the population. The possible role of density dependence is discussed.