Supernumerary chromosomes in different families of Decapod Crustaceans

SUPERNUMERARY CHROMOSOMES IN DIFFERENT FAMILIES OF DECAPOD CRUSTACEANS. Salvadori S.*, Deiana A.M.*, Coluccia E.*, Milia A.**, Cannas R.* & Cau A*. Dipartimento di Biologia Animale ed Ecologia*; Dipartimento di Biologia Sperimentale**- Universita’ di Cagliari, Viale Poetto 1, 09126 Cagliari, Italy. Supernumerary (B) chromosomes are additional chromosomes, highly polymorphic in number and morphology within populations and species. They show non-Mendelian heredity, can be monosomic or disomic and often asynaptic. B chromosomes are widespread in both animals and plants, and “have been found in all groups of organisms citologically well studied” (Jones and Rees, 1982). The biological role of these chromosomes has been discussed for a long time; several experimental data show that they could affect the fitness of the carrier. Their presence has been also hypotesized in some species of Decapoda because of the remarkable numerical variability in both mitotic and meiotic chromosome counting, but without a cytological demonstration (Roberts, 1969; Hughes, 1982; Corni, 1989). In the present study we provide evidence of the presence of chromosomes with some of the features and characteristic behaviour of B chromosomes (Jones & Rees, 1982; Jones, 1985) in two Decapod species, Palinurus elephas and Nephrops norvegicus, belonging to the Palinuridae and Nephropidae families respectively. Chromosome preparations were obtained from testicular tissue using a modification of the air-drying technique. C- and fluorochrome- banding and restriction enzyme-induced banding (RE) allowed a sharp differentiation and a clear identification of chromosomes with features typical of B chromosomes in terms of heterochromatin content, asynapsis and distorted segregation. The presence of these chromosomes can, at least partially, account for the numerical variability already observed in the karyotype of these species. In Nephrops norvegicus C-banding as well as Dde I-induced banding identified three groups of B chromosomes with different features. Some of them were the largest chromosomes of the complement. C- and DdeI- banding showed that one/two of these chromosomes are only partially heterochromatic, while all the others are completely heterochromatic. Q-banding was useful to show intercalary and subtelomeric bands in the large B chromosomes as well as entirely Q-positive short arms in some small submetacentric chromosomes. In meiotic division I, B chromosomes appeared tightly condensed and asynaptic. Distorted segregation was indicated in metaphases II from the same meiocyte by their random distribution. (Deiana et al., 1996). In Palinurus elephas, C-banding and HaeIII-induced banding revealed the presence of small B chromosomes, completely heterochromatic and variable in number. In meiosis I, they have been observed forming bivalents, univalents and multivalents. In Decapoda, B chromosome polymorphism could be a useful marker in investigating the variability among allopatric populations. CORNI M.G., TRENTINI M. & FROGLIA C., 1989. Karyological study on Nephrops norvegicus (L., 1758)(Astacidea, Nephropidae) in the Central Adriatic sea. Nova Thalas¬sia, 10, 127-131. DEIANA A.M., COLUCCIA E., MILIA A., SALVADORI S., 1996. Supernumerary chromosomes in Nephrops norvegicus L. (Crustacea, Decapoda). Heredity, 76, 92-99. HUGHES J.B., 1982. Variability of chromosome number in the lobsters, Homarus americanus and Homarus gammarus. Caryologia, 35(2), 279-289. JONES R.N. & REES H., 1982. In: B chromosomes, 19-68 (Acade¬mic Press, New York). JONES R.N., 1985. Are B-chromosomes selfish?. In: Cavalier¬Smith T. (ed.), The evolution of genome size, John Wiley & Sons, 397-425. ROBERTS F.L., 1969. Possible supernumerary chromosomes in the Lobster, Homarus americanus. Crustaceana, 16, 194-196.