Bibliography: pages 93-108.The aim of this thesis was to isolate and characterize a homeobox-containing gene of the South African sea urchin Parechinus angulosus. This was achieved by constructing a genomic library of several individuals and screening this library using a probe containing the Antennapedia homeobox. Eight clones were isolated and shown to represent different alleles of the same gene. One clone was sequenced, revealing a homeobox which was termed PaHboxl. This homeobox was compared to published homeobox sequences and shown to be a member of the Antp (Hoxl.l) subclass (table 1.1). A splice donor site was identified 23 bp upstream of the homeobox and the observation confirmed by RNAase mapping. PaHboxl is situated in a genomic area showing a significantly higher degree of restriction fragment polymorphism than expected. This was shown by a statistical analysis which should be of general value in the interpretation of such polymorphisms. The expression of PaHboxl was examined by RNAase protection assays and Northern blotting. Two distinct phases of expression were observed - during embryogenesis PaHboxl is expressed transiently at low levels in 11,5 hr mesenchyme blastula stage embryos (44 ± 8 transcripts per embryo) with levels 3-5 fold lower 2,5 hr before and after this stage. Expression is observed again at up to 160 fold higher levels in the adult with maximal expression in testis (11 transcripts per 10 pg total RNA), and increasingly lower levels in intestines, ovary and Aristotle's lantern. Two transcripts of size 5,2 and 5,7 kbp were observed. Expression in Aristotle's lantern and embryonic stages could not be detected by Northern analysis
