Chromosome number of sea urchin andromerogones during early development

Abstract

The chromosome number of the andromerogones obtained by fertilizing non-nucleate egg fragments of Hemicentrotus pulcherrimus was examined by an air-drying method during early development. The non-nucleate egg fragments were prepared by centrifuging unfertilized eggs in a stepwise saccharose density gradient with a purity of 99.9%, comprising from about one-tenth to one-fourth of the total egg volume. The andromerogones cleaved, hatched and then actively swam. Though their development tended to be slower compared with that of control whole embryos after hatching, most of them developed into larvae and the remainder developed as permanent blastulae. While the larvae had differentiated gut and pigment cells, they also had various skeleton types which varied from almost normal to irregular. The chromosome preparations were made from blastomeres dissociated from many andromerogones. The rate of cells having a haploid chromosome number of 21 was 73% at the two cell stage, 84% at the 8 cell stage, 75% at the morula stage, 76% at the hatching blastula stage and 87% at the swimming blastula stage, while that of cells having a diploid number of 42 was 2% if averaged out for total cells examined. A total of 73-87% of the andromerogones was found to develop with a haploid number of chromosomes.