Chromosome and karyotype of the crayfish (Procambarus clarkii)

The crayfishProcambarus clarkii was introduced into China in the last century and has become a commercially important cultured species. Most of previous research of P. clarkii focus on physiology, ecology and reproductive biology. Little is known about its genetic basis. Chromosomes are carriers of genetic material. The study of chromosome inP. clarkiican provide indispensable basic data for genetic improvement and breeding of new varieties. In this paper, the chromosome and karyotype of the crayfish P. clarkiiwere studied by preparing metaphase spreads from the tissue of immature testis using the conventional method of hypotension and air-dry. Well-spreads in meiotic metaphase of spermatocyte and mitotic metaphase of spermatogonium were obtained after DAPI and Giemsa staining. Most of chromosomes in mitotic metaphase were X-shaped, which are regarded as metacentric chromosomes. The bivalents in meiotic metaphase were short rod with heterochromatin located at the end. The number of chromosomes in mitotic metaphase and bivalents in meiotic metaphase was statistically calculated based on 50 spermatocytes and 26 spermatogonia division phases, respectively. The number of bivalents which was N=94 appeared 28 times, accounting for 56% of spermatocyte. The number of chromosomes with 2N=188 appeared 20 times, accounting for 77% of the number of mitotic spermatogonia. Thus we concluded that the number of diploid and haploid chromosomes of P. clarkiiwas 2N=188 and N=94, respectively. Since the mitotic chromosome is too small to perform karyotype analysis, we conducted a preliminary meiotic karyotype analysis based on bivalent length and position of the centromere. The karyotype formula is N=55M+22(SM, ST)+17T, including 55 metacentric bivalents, 22 sub-metacentric or acrocentric bivalents, and 17 telocentric bivalents. No sex chromosome was identified.