An unusual evolutionary behaviour of a sea urchin histone gene cluster

Abstract

DNA sequences of cloned histone coding sequences and spacers of sea urchin species that diverged long ago in evolution were compared. The highly repeated H4 and H3 genes active during early embryogenesis had evolved (in their silent sites) at a rate (0.5‐0.6% base changes/Myr) similar to single‐copy protein‐coding genes and nearly as fast as spacer DNA (0.7% base changes/Myr) and unique DNA. Thus, evolution in the major histone genes conforms to a universal evolutionary clock based on the rate of base sequence change. By contrast, the H4 and H3 coding sequences and a non‐transcribed spacer of the DNA clone h19 of Psammechinus miliaris show an exceptionally low rate of sequence evolution only 1/100 to 1/200 that predicted from the clock hypothesis. According to the classical model of gene inheritance, the h19 DNA sequences in the Psammechinus genome require unusual conservation mechanisms by selection at the level of the gene and spacer sequences. An alternative explanation could be recent horizontal gene transfer of a histone gene cluster from the very distantly related Strongylocentrotus dröbachiensis to the P. miliaris genome.

Intra- and interspecies analyses of the carcinoembryonic antigen (CEA) gene family reveal independent evolution in primates and rodents

“…1987). This is consistent with the a s s u m p t i o n that there should be negligible selective pressure on both kinds of sites (Miyata et al 1980;Perler et al 1980;Busslinger et al 1982).…”

“…Furthermore, we determined the rate of Substitutions in the intron regions (KN) flanking the N-terminal d o m a i n exons of several h u m a n CEA gene family members. The value o f Ks can be used to estimate the absolute or relative branching points of phylogenetically related sequences (Busslinger et al 1982; S a k o y a m a et al 1987). Usually, the values for Ks and KN found in the literature are approxi- Fig.…”

A cytomegalovirus DNA sequence containing tracts of tandemly repeated CA dinucleotides hybridizes to highly repetitive dispersed elements in mammalian cell genomes.

“…However, in both cases, these represent aberrant defective genomes requiring helper viruses for propagation, whereas all of the examples of cell-virus homology that we have described in herpesvirus genomes occur in the wild-type infectious genomes. We note that Busslinger et al (6) have made the interesting suggestion that DNA viruses may have been the agents for the apparent recent transfer of histone genes from one species of sea urchin to another noninterbreeding species. …”

Characterization of two nonallelic pairs of late histone H2A and H2B genes of the sea urchin: differential regulation in the embryo and tissue-specific expression in the adult.

“…This demonstrates that the differential expression of late H2A and H2B variants during embryogenesis must have already existed 65 million years ago in the common ancestor of both sea urchin species (11). S1 nuclease analysis of steady-state mRNA did not allow us to distinguish whether the observed differential expression pattern is primarily brought about by transcriptional or posttranscriptional regulation.…”