Two studies have provided separate pieces of information
that bear on the functional and evolutionary significance
of introns in the budding yeast Saccharomyces cerevisiae
(Holstege et al., 1998; Spingola et al., 1999). By the
measure of the number of introns within its genes, budding
yeast seems a disappointing eukaryote. Fewer than 250 of
the more than 6,200 annotated genes are known to have introns,
and fewer than 10 are known have more than one intron (Spingola
et al., 1999). In contrast, metazoan genes are estimated
to average nearly 10 introns, and the intronless gene is
the exception rather than the rule. Although many essential
yeast genes have introns, it would appear that introns
are on the way out of the yeast genome, perhaps by a cDNA-directed
homologous recombination mechanism proposed by Fink (1987).