MEIOTIC CHROMOSOME COMPLEMENTS AND NUCLEAR DNA CONTENTS OF FOUR SPECIES OF SHRIMPS OF THE GENUS PENAEUS

Abstract

Meiotic chromosome spreads were prepared from testicular lobes of 4 species of penaeid shrimps to determine numbers of chromosomes and meiotic phases. DNA content of nuclei isolated from shrimp hemocytes of these species was estimated by flow cytometry. Meiotic chromosome spreads indicated that the haploid chromosome number of Penaeus aztecus, P. duorarum, and P. vannamei was 44, while that ofP. setiferus was 45. These counts are consistent with diploid chromosome numbers reported previously for each of these species. No trivalents or quadrivalents were found in meiotic prophase. These 4 species had approximately the same genome size (approximately 70% of the human genome) as measured by flow cytometry. It is suggested that Robertsonian chromosome rearrangement has accompanied the speciation process in the genus Penaeus without polyploidization. Chromosome research in the decapod Crustacea has advanced slowly (Farmer, 1974; Milligan, 1976; Hughes, 1982; Hayashi and Fujiwara, 1988). Milligan (1976) and Hayashi and Fujiwara (1988) provided new techniques to obtain mitotic chromosome metaphase preparations in penaeid shrimps, but these works are still concerned only with chromosome counts and comparison of chromosome numbers between species. Thus, no karyotype has ever been described in this animal group. Although the small size and large numbers of chromosomes present some difficulty, determination of the genome size and examination of meiotic chromosome phase may provide additional information. In this paper, we report observations on meiotic chromosome phases in 4 species of shrimps of the genus Penaeus, with determinations of their genome size.