Current knowledge on B chromosomes in natural populations of helminth parasites: a review

Helminths, traditionally classified into three phyla Platyhelminthes, Nemathelminthes and Acanthocephala, are a phylogenetically broadly diversified group of invertebrates, characterised by a parasitic life style. Current estimates of the helminth species diversity are at least 23-40,000 platyhelminthes, 10-26,000 nematodes and 1,200 acanthocephalans. Recent information on helminth karyotypes is fragmentary, and basic karyological data are known from approximately 1.1% of known species. Supernumerary chromosomes have been reported in selected populations of only 11 digenean flukes (Platyhelminthes), 1 thorny-headed worm (Acanthocephala) and 4 roundworms (Nematoda), which represent 3.6, 7.7 and 1.3% of the total number of species cytogenetically analysed to date within respective helminth groups. B chromosome presence was not generally associated with heteromorphic sex chromosomes as they occurred both in hermaphroditic flukes and dioecious helminths, and in species having male or female heterogametic sex chromosomes (ZW of schistosomes, XO of acanthocephalans and XY of nematodes). Numbers of B chromosomes varied from 1 to 10. Most often, Bs represented one or two of the smallest elements of the complement but they could be much bigger in some digenean flukes. B chromosomes showed a diverse morphology, including telocentric to metacentric structure. There is no detailed banding or ultrastructural study of Bs in the majority of helminth carriers. Assumptions on the possible relation between the occurrence of Bs in endoparasitic helminths and extreme environments are discussed.