Towards a reconciliation of the introns early or late views: triosephosphate isomerase genes from insects.

The gene encoding the glycolytic enzyme, triosephosphate isomerase (TPI; EC 5.3.1.1), is a favourite model for molecular evolutionists who either subscribe to the theory that introns co-evolved with the ancestral gene, the introns early view, or alternatively, that introns are more recent immigrants. The discovery of an intron in the TPI gene of Culex mosquitoes at a site which was predicted by proponents of the intron early school supported that theory. More recently, the discovery of additional intron sites in several eukaryotes was presented as evidence supporting the introns late school. We have found the 'Culex intron' in two closely related mosquitoes, but not in two more evolutionary primitive Dipterans, suggesting that, if it is an 'ancient intron', loss may be more frequent than that supposed by the intron late school. In addition, we have found that three introns punctuating the TPI gene from the Lepidopteran, Heliothis, appear to be ancestrally related and may be the result of transposable element insertion, 50-90 million years ago. It is argued that both opposing schools in the intron debate be reconciled -- some introns may have been early and certainly others have arrived subsequent to the appearance of the TPI gene.