CHROMOSOMES AND NUCLEAR-DNA OF CRUSTACEA

Present knowledge about chromosomes and nuclear DNA of Crustacea is re viewed. Haploid chromosome numbers range from 3 (Acanthocyclops) to 18 8 (Astacus). Chromosomes are generally small to medium in size (1-5 mu m) and punctiform or rod-shape. Nuclear DNA amounts show a large rang e, lying between 0.37 (Daphnia) and 22.6 pg (Decapoda) per haploid gen ome, 5.5 x 10(8) nt to 1.8 x 10(10) nt. Highly repetitive sequences ma y represent as much as 30% of the genome and show a high degree of con servation in Brachyura, while some intermediate repetitive sequences a re under-represented. The relationship between taxonomy and cytogeneti cs appears very complex in some taxa, such as Artemia (Branchiopoda). This genus includes bisexual sibling species with female heterogamety and parthenogenetic populations with different levels of ploidy, up to triploid and pentaploid. Daphnia presents diploid and polyploid strai ns, and parthenogenetic reproduction. Chromatin diminution occurs in C opepoda; in this group male and female heterogamety coexist with a ZO- ZZ type, unique among animals. Evolution in Copepoda tends to the redu ction of chromosome number. Some parthenogenetic, freshwater Ostracoda have supernumerary chromosomes; Ostracoda also present complex sex ch romosomes mechanisms (XO and XY, with cases of multiple X's and multip le Y's). Within Rhizocephala, chromosome numbers can be used to distin guish species of parasitic Crustacea, such as Sacculina. Robertsonian translocations are known in some Isopoda. Jaera albifrons albifrons pr esents a dine, diminishing from north (n=13) to south (n=9) along N-E European coasts. As in Copepoda, female and male heterogamety are pres ent. Cytogenetics of Amphipoda is not well known, and many species hav e the same number; as much as 9 supernumerary (B) chromosomes have bee n reported. One of the highest numbers of chromosomes known for animal s occurs in crayfish Astacus (2n=376). DNA amount in shrimps has the h ighest range in Crustacea (3-22 pg/c) and one of the highest for anima ls. Within several Decapoda groups, viz. lobsters, crayfish, scyllarid s and hermit crabs, polyploidy may have acted as an evolutionary facto r. Chromosome evolution within Decapoda is difficult to understand bec ause the chromosomes are numerous, very small and punctiform; differen tiated sex chromosomes have sometimes been reported, the male being th e heterogametic sex. Somatic endopolyploidy is present in many groups mainly in the digestive tract; polyploidy in germ cells might have occ urred in different groups during evolution.