Evolution of chorion gene families in lepidoptera: characterization of 15 cDNAs from the gypsy moth.

Fifteen unique chorion protein-encoding cDNAs from gypsy moth have been completely sequenced. These sequences are encoded by a family of genes, based on pairwise similarity values of 78-100% within a 225-nt region. Pairwise comparisons and maximum parsimony analysis strongly support the existence of two clusters of 11 and four sequences each, called noc1 and noc2. While noc2 consists of two subclusters, there is little character support for subclusters within noc1. The highly localized character-state distribution on the parsimony tree in gypsy moth is reminiscent of that in Bombyx mori, specifically for those chorion families that have been shown to undergo gene conversion. Gene conversion thus becomes a reasonable explanation for the homogeneity of noc1 sequences and for their distinctness from noc2. The relationship between the two major clusters of chorion sequences in gypsy moth (noc1, noc2) and Bombyx mori (Bm alpha, Bm beta) has been addressed through mixed-species tree construction. All four groups cluster separately, thus providing no direct evidence of orthologous sequences. However, the occurrence of gene conversion could have eliminated such evidence. The relationship between the chorion gene tree and the species cladogenic event is discussed, as are biases in codon usage, base composition, and nucleotide transformations.