This abbreviation describes the sex chromosome constellation of many male nematodes. This pattern means that the male has only one sex chromosome (e.g., Trichuris trichiura). While the female has in total 2n = 8 chromosomes, males of this species possess 2n = 7 chromosomes. However, males of other species in the same genus may be of the XY type and have two sex chromosomes (e.g., T. ovis, which show 2n = 5). The following species also belong to the XO type: Contracaecum spiculigerum (2n = 16), Strongylus edentatus (2n = 12), Haemonchus contortus (2n = 12), Ancylostoma caninum (2n = 12), and Setaria equina (2n = 12).
Also ixodid ticks have developed the XO type, so that the males possess one chromosome less than the females (e.g., Haemaphysalis sulcata: ♀ = 22, ♂ = 21). On the other hand, leather ticks are of the XX-XY type, so that both sexes possess always the identical number of chromosomes (e.g., ♀, ♂ of Ornithodoros gurneyi: 12; O. alactagalis: 34; Argas sp.: 26).