The adaptive and evolutionary significance of wing polymorphism and parthenogenesis in Ptinella Motschulsky (Coleoptera: Ptiliidae)

Abstract

Abstract. The incidence of wing polymorphism varies seasonally in field populations of Ptinella aptera and Ptinella errabunda with alatae increasing in abundance in summer. In laboratory cultures of P. aptera alate development varies with temperature. This implies that morph determination is based on a physiological switch sensitive to temperature and possibly other adaptively relevant environmental factors. Alate P. aptera produce a higher proportion of alate offspring than do apterous parents suggesting an inherited component in polymorphism control, or some form of maternal effect. The female biased sex ratio expected of a regularly inbreeding species was found only in the alate morph. Apterous beetles show a 1:1 sex ratio which is explained by the females' limited capacity to store sperm, the enormous size of the sperm, and the consequent need for multiple insemination. Ptinella errabunda is thelytokous but fails to realize the potentially doubled fecundity associated with parthenogenesis. Both the number of eggs produced and their viability is lower than in bisexual congeners. Enhanced colonizing ability, rather than energy economy, is considered to be the most important selective advantage of parthenogenesis to Ptinella in the subcortical habitat. The spatial and temporal heterogeneity of the subcortical habitat is considered to have favoured the evolution and maintenance of parthenogenesis and wing polymorphism in Ptinella.