CHANGES IN THE PATTERN OF TWISTED GASTRULATION GENE-EXPRESSION AMONG DROSOPHILA SPECIES

A long-standing hypothesis posits that morphological changes may be mo re likely to result from changes in regulation of gene expression than from changes in the protein coding sequences of genes. We have compar ed the expression pattern of the twisted gastrulation (tsg) gene among five Drosophila species: D. melanogaster, D. simulans, D. subobscura, D. mojavensis, and D. virilis. The tsg gene encodes a secreted protei n that is required for the specification of dorsal midline fates in th e Drosophila early embryo. TSG is unlike other secreted growth and dif ferentiation factors in Drosophila in that its expression pattern can be experimentally varied and still result in normal development. Becau se of this, its regulatory region may be freer to diverge than that of other developmental genes whose misexpression may lead to lethal defe cts. Thus, the tsg gene may be a good indicator of the frequency and n ature of evolutionary changes affecting patterns of gene expression. O ver similar to 60 million years (Myr), the tsg gene has retained a dor sal-on/ventral-off pattern and a middorsal region of expression; but t here have been marked changes in the middorsal domain of expression as well as the appearance/loss of other domains of expression along the anterior/posterior axis. Changes between closely related species (simi lar to 2-5 Myr since divergence) that are not reflected among more dis tantly related species suggest frequent changes in gene expression ove r evolutionary time. These changes in gene expression may serve as the raw material for eventual evolutionary changes in morphology.