High-resolution analysis of karyotypes prepared from different tissues of the eastern oyster Crassostrea virginica

Establishment of chromosome identity is the first step of physical genome m apping. This step is hindered by a lack of banding techniques and size homo geneity in the chromosomes of the eastern oyster, Crassostrea virginica. In this study, chromosomes prepared from adult gill, ripe gonad tissues, and embryos were analyzed with a computer-based measurement system. Chromosomes from embryos were elongated with excellent morphology and identifiable sec ondary constrictions, although homologous pairs were difficult to establish because of asymmetric sizes found between homologues. Meiotic chromosomes at the stages of diakinesis (testis) and pachytene (ovary) offered distinct advantages for karyotyping. These chromosome bivalents possessed a haploid chromosome number (n = 10) with transverse chromomere bands analyzable by microdensitometry. Chromosomes derived from gill tissue were highly condens ed and few spreads were analyzable. Idiograms of each chromosome were devel oped in this study based on size, centromere position, and chromomere bands . These results indicate that mitotic and meiotic cells are each important for the study of chromosomes of the eastern oyster and that computer-assist ed analysis will be useful for establishment of karyotypes and idiograms.