High Diversity of Karyotypes, Allozymes, and Life History Traits in Spruce Sawflies of the Genus Cephalcia (Hymenoptera : Pamphiliidae)

Seven species of Cephalcia spruce sawflies sampled in Europe and China differ considerably in some life history traits such as prepupal weight, time of adult emergence, clustering of the eggs and larval gregariousness. Genetic divergence detected by allozyme analysis was substantial, even among siblings or closely related forms. Haploid chromosome numbers range from n=18 to n=35 in Cephalcia, whereas the related sawfly Acantholyda erythrocephala has n=11. Within the C. arvensis complex, geographical and phenological (adult emergence time) forms show chromosome numbers from n=23 to n=29. As a general tendency, low numbered karyotypes are metacentric rich while the high numbered are acrocentric rich. The genetic variation pattern among species show little concordance with the chromosome differences. Some species with low genetic distance have very different karyotypes, whereas other species with similar karyotypes are genetically rather distant. These chromosome differences are likely to be related to natural history traits and population structure affecting the fixation rate of chromosome mutants. In the two largest and gregarious species, C. abietis and C. alashanica, chromosome evolution seems to be directed toward increasing chromosome number but karyological changes seem to be rather rare events. Conversely, in smaller species, C. arvensis complex and the siblings C. alpina and C. annulicornis, with solitary larval behaviour and large variability in phenology, chromosomal changes seem to be fixed much more frequently. In particular, phenological isolation of breeding units with a small effective population size, enhanced by the haplodiploid genetic system, could favour the stochastic fixation of rearrangements, and chromosomally differentiated taxa may arise.