Karyology of pseudogamous Ips bark beetles

Females in unisexual strains of North American Ips tridens, I. borealis, I. pilifrons, and I. perturbatus and I. acuminatus from Norway and Bulgaria are triploid (3n=48) and are associated with males and diploid (2n=32) females that produce bisexual broods. Triploid females require insemination for brood production, but male‐contributed genes do not appear in their offspring: inheritance of morphological markers (frons shape and setation) in North American spp. is strictly mother‐to‐daughter, as has been shown for isozymes in I. acuminatus. These unisexual Ips strains are therefore pseudogamous (=gynogenetic). For the North American spp., morphological markers and the prevalence of all‐female strains in natural populations indicate that pseudogamy has arisen sporadically. Triploid lines may arise by fertilization of diploid ova by normal sperm. The occurrence of diploid ova was inferred from observations of putatively diploid and tetraploid spermatozoa associated with distortions in meiosis of hybrid males and of nonhybrid males affected by apparently genetically‐controlled asynapsis at first meiosis.