Chromosomes and nuclear DNA of Crustacea

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Abstract
Present knowledge about chromosomes and nuclear DNA of Crustacea is reviewed. Haploid chromosome numbers range from 3 (Acanthocyclops) to 188 (Astacus). Chromosomes are generally small to medium in size (1–5 μm) and punctiform or rod-shape. Nuclear DNA amounts show a large range, lying between 0.37 (Daphnia) and 22.6 pg (Decapoda) per haploid genome, 5.5 × 108 nt to 1.8 × 1010 nt. Highly repetitive sequences may represent as much as 30% of the genome and show a high degree of conservation in Brachyura, while some intermediate repetitive sequences are under-represented. The relationship between taxonomy and cytogenetics 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 strains, and parthenogenetic reproduction. Chromatin diminution occurs in Copepoda; in this group male and female heterogamety coexist with a ZO-ZZ type, unique among animals. Evolution in Copepoda tends to the reduction of chromosome number. Some parthenogenetic, freshwater Ostracoda have supernumerary chromosomes; Ostracoda also present complex sex chromosomes mechanisms (XO and XY, with cases of multiple X's and multiple Y's). Within Rhizocephala, chromosome numbers can be used to distinguish species of parasitic Crustacea, such as Sacculina. Robertsonian translocations are known in some Isopoda. Jaera albifrons albifrons presents a cline, diminishing from north (= = 13) to south (n = 9) along N-E European coasts. As in Copepoda, female and male heterogamety are present. Cytogenetics of Amphipoda is not well known, and many species have the same number; as much as 9 supernumerary (B) chromosomes have been reported. One of the highest numbers of chromosomes known for animals occurs in crayfish Astacus (2n = 376). DNA amount in shrimps has the highest range in Crustacea (3–22 pg/c) and one of the highest for animals. Within several Decapoda groups, viz. lobsters, crayfish, scyllarids and hermit crabs, polyploidy may have acted as an evolutionary factor. Chromosome evolution within Decapoda is difficult to understand because the chromosomes are numerous, very small and punctiform; differentiated sex chromosomes have sometimes been reported, the male being the heterogametic sex. Somatic endopolyploidy is present in many groups mainly in the digestive tract; polyploidy in germ cells might have occurred in different groups during evolution.