Interspecific Mating and Cytological Studies of Closely Related Species of Ips DeGeer and Orthotomicus Ferrari (Coleoptera: Scolytidae)

Abstract

Interspecific pairing of closely related species of Ips and Orthotomicus was undertaken to test their degree of breeding isolation. Pairings of the following species frequently resulted in sperm transfer but no eclosion: Ips mexicanus × I. concinnus (group I), confusus × montanus (group IX), plastographus (group III) × pini (group IV) and Orthotomicus latidens × O. sabinianae. Mating was generally unsuccessful when the more distantly related species, confusus × plastographus and confusus × pini, were paired. Individuals which produce all-female broods were discovered in a population of O. latidens. About 50% of the eggs deposited by these females hatched, whereas 92% of the eggs from “normal” females produced larvae. Cytological investigations revealed no constant differences between the karyotypes of I. mexicanus and concinnus (7AA+Xyp), O. latidens and sabinianae (9AA+Xyp), or between I. confusus and montanus (15AA+Xyp). I. pini (oregonis) and plastographus also have karyotypic formulae of 15AA+Xyp, but both are easily distinguished from confusus and montanus and an atypical chromosome in pini (oregonis) distinguishes it from plastographus. The mexicanus and concinnus karyotype and that of O. latidens and sabinianae can be directly derived from each other by fusion or fission. Their relationship to the karyotype of the European species O. proximus (6AA+Xy), is plausible but their karyotypes are composed of a remarkably different number of autosomal arms from those known for other species of Ips.