Extinctions in the Hymenoptera: genes, behaviour, and the dynamics of bottlenecked populations – SEXTINCTION

Most populations deserving the elaboration of management practices are bottlenecked populations. Such populations are experiencing a drastic decrease in population size and are therefore affected by the specific processes occurring with such a decrease. These populations include declining populations of threatened species, fortuitously introduced populations (biological invasions), and purposely introduced populations (the introductions of natural enemies for the biological control of pests, the reintroductions and translocations of threatened organisms for biological conservation). There is ample motivation to develop the research on small populations. Notably the three following problems are both very important and actual: (1) the worldwide decrease in biodiversity; (2) menaces from biological invasions; and (3) the urgent need to decrease the use of pesticides and hence, to improve biological control strategies.

Different processes affect the biology of bottlenecked populations: (1) demographic stochasticity, (2) decrease in genetic variability and subsequent adaptability to environmental changes, (3) the Allee effect, i.e., the difficulty for individuals to survive and/or reproduce when rare and the subsequent low or negative population growth rate. The relative weight of these different processes, especially concerning genetic and demographic processes, is being discussed in a passionate body of literature. Besides, the Allee effect is emerging as a broad theoretical frame which has the advantage of considering any mechanism which affects fitness in small populations. Such a frame could play a major role in federating researches on the biology of small populations.

Our project relies on a multidisciplinary approach of the population biology of bottlenecked populations of hymenopterous organisms. These insects have a sex determination genetic system (sl-CSD for single locus complementary sex determination) which yields severe consequences for the adaptation of behavior and the genetics and dynamics of small populations. The problem rises because homozygous individuals at the unique sex determination gene develop into diploid males, which are sterile (normal diploids, among the hymenoptera, develop into females). Consequently, the expected scenario in a bottleneck is: decrease in genetic variability ? increase in the proportion of diploid sterile males ? decrease in the rate of population increase ? extinction. This scenario is referred to as the “diploid male vortex”; it is a genetic-demographic positive feedback which yields expected extinction rates in the Hymenoptera an order of magnitude higher than extinction rates observed in diploids with comparable traits (Zayed & Packer, PNAS, 2005).

Despite the huge implications of the diploid male vortex to approach the population decline which is being observed among pollinating wasps, or the low establishment rates of introduced populations of biological control agents, the diploid male vortex has not been assessed yet with thorough research.

Our project is based on the association of four research teams having complementary expertise to approach the problem in a common species, the hymenopterous parasitoid Venturia canescens. This association will allow integrating (1) molecular genetic mechanisms based on the nature and the expression of the CSD gene, (2) consequences of inbreeding depression resulting from the CSD on behavioral decisions (and more specifically, mate-choice) and the evolution of chemical volatile signatures allowing such decisions, (3) the effect of bottleneck and habitat fragmentation on the genetic and demographic dynamics of populations, and (4) the consequences of CSD on host-parasitoid or prey-predator specific dynamics, and on the control of hosts or preys.