Assortative mating and intrapopulation polymorphism maintenance in natural populations and laboratory lines of insects

Speciation is a microevolutionary process that starts with intraspecies differentiation, followed by the establishment of reproductive isolation. One of the intrinsic isolating factors causing physiological isolation is assortative mating. As a result of many years of observations of the natural populations on the territory of the South Urals (the Bashkortostan Republic) and long-term laboratory experiments, we revealed assortative mating in populations of the potato beetle Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae). The assortativity of mating manifested itself as a statistically significant deviation from the normal distribution of the frequencies of imago pairs belonging to three morphological types, which are defined by the degree of integument pattern melanization, namely, the achromists (A), melanists (M), and intermediate type (I). In copulo, we have not detected mating between achromist males (A) and melanist females (M), with the active choice in this case belonging to males. Under the laboratory conditions, with a sampling of 40 artificially formed pairs, we revealed statistical differences in the longevity and breeding performance of imagoes belonging to different morphological types. Achromists and melanists were characterized by a statistically higher longevity, almost two times higher than that in individuals of the intermediate type. Females of the intermediate type showed statistically higher fecundity in the homonomous crosses than achromists and melanists. It was demonstrated that the viability of the progeny also differed statistically in different pairs. The highest viability was detected for the progeny obtained from the type A and type I females. The observed differences indicated the existence of different reproductive strategies in studied species populations. In the S laboratory line of the common house fly Musca domestica L. (Diptera: Muscidae), we detected individuals with different reproductive strategies associated with longevity. The maximum longevity in two inbred lines, Sh28 (short-living) and L2 line (long-living), isolated from the S line showed a statistical, almost twofold difference. Mass reproduction time is confined to the first or second week of the imago stage in the short-living fly line; it is shifted to the third or fourth week in long-living flies. In reciprocal crosses between the lines, we detected a dramatic decrease in the fecundity of the females and a decrease in the viability of the hybrid females. The results confirm the essential role of assortative mating and frequency-dependent selection in the maintenance of the intrapopulation diversity.