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

Abstract

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.

Comparative karyological analysis of three members of Allocreadiidae (Digenea): taxonomic and phylogenetic implications

“…Intraspecific chromosome number variation was detected in 11 digenean species due to the presence of supernumerary B chromosomes (for review, see Špakulova and Casanova 2004). However, B chromosomes were never found in species of higher Plagiorchiida, and this phenomenon could not account for the variation in chromosome number in B. luciopercae.…”

Molecular Cytogenetics in Digenean Parasites: Linked and Unlinked Major and 5S rDNAs, B Chromosomes and Karyotype Diversification

“…Bs have been described in 12 digenean species belonging to 7 families [Špakulová and Casanova, 2004;Petkevičiūtė et al, 2012]. In most of these species, the number of Bs detected in metaphase plates predominantly ranged from 0 to 2 [Špakulová and Casanova, 2004;Petkevičiūtė et al, 2012], but Diplodiscus subclavatus [Petkevičiūtė et al, 1989] and Notocotylus sp . [Baršiené et al, 1990] displayed 0-10 Bs.…”

“…; however, this approach has been scarcely applied to Digenea, and chromosome analyses have been performed on only about 300 of the 18,000 described species [Baršiené, 1993;Bell et al, 1998;Petkevičiūtė et al, 2003;Špakulová and Casanova, 2004;Reblánová et al, 2011;Zadesenets et al, 2012a, b;Petkevičiūtė et al, 2012Petkevičiūtė et al, , 2014Petkevičiūtė et al, , 2015Sofi et al, 2015]. Most of them were restricted to the description of diploid chromosome numbers and karyotypes obtained from Giemsa-stained metaphase plates [Baršiené, 1993;Špakulová and Casanova, 2004].…”

“…Most of them were restricted to the description of diploid chromosome numbers and karyotypes obtained from Giemsa-stained metaphase plates [Baršiené, 1993;Špakulová and Casanova, 2004]. Although some of the aforementioned studies allowed the detection of both alterations in the ploidy level [Petkevičiūtė et al, 2014] and the presence of supernumerary B chromosomes [Špakulová and Casanova, 2004;Petkevičiūtė et al, 2012], most structural chromosome reorganizations would remain unidentified after Giemsa staining. As in many other invertebrate species, accurate chromosome identification in digeneans requires sets of specific chromosome markers detectable by FISH, and tandemly repeated multigene families are the best candidates.…”

Probing the W chromosome of the codling moth, Cydia pomonella, with sequences from microdissected sex chromatin

“…Female heterogamety is common in vertebrates, as it is conserved in birds (Pigozzi 1999;Ellegren 2000) and snakes (Jones and Singh 1985) and also occurs in some lizards, amphibians, and fish (Wallace et al 1999;Graves and Shetty 2001;Schmid and Steinlein 2001;Schartl 2004). In invertebrates, however, it is known only in two sister orders of insects, the Trichoptera (caddis flies) and Lepidoptera (moths and butterflies; Traut 1999), and in parasitic fluke worms of the family Schistosomatidae (Špakulová and Casanova 2004). Heterogametic females were also reported from a few plant species, but this remains to be confirmed (Vyskot and Hobza 2004).…”