Abstract
A new developmental gene family, recently identified in D. melanogaster, has been called imaginal disc growth factors (IDGF) because the proteins promote growth of cell lineages derived from imaginal discs. These are the first genes reported that encode polypeptide factors with mitotic activity in invertebrates. Characteristics such as similar arrangement of introns and exons, small size, and different cytological localization make this family an excellent candidate for evolutionary studies. We focus on the loci Idgf1 and Idgf3, two genes that possess the most distinctive features. We examine the pattern of intra- and interspecific nucleotide variation in the sequences from 20 isogenic lines of D. melanogaster and sequences from D. simulans and D. yakuba. While MK, HKA, and Tajima's tests of neutrality fail to reject a neutral model of molecular evolution, Fu and Li's test with outgroup and McDonald's test suggest that balancing selection is modulating the evolution of the Idgf1 locus. The rate of recombination between the two loci is high enough to uncouple any linkage disequilibrium arising between Idgf1 and Idgf3, despite their close physical proximity.
Highlights
A new developmental gene family has been identified in Drosophila melanogaster (Kawamura et al 1999)
We applied several different methods to obtain a phylogenetic tree—neighbor-joining, maximum parsimony, as well as maximum likelihood (HKY-gamma substitution model), which were conducted on amino acid and DNA sequences
Evolution of the imaginal disc growth factors (IDGF) gene family: Our observations corroborate that the Idgf genes in D. melanogaster form a small gene family with six members, which has been previously indicated by their homology at the DNA and protein level and by similar functions
Summary
The other genes of the family, we used GenBank sequences Polymerase chain reaction (PCR), cloning, and nucleotide sequencing: For each line an ف2.0-kb region encompassing the Idgf transcriptional unit was amplified using two primers (forward primer 5Ј-TGCAGACCCCTAAAAGTTGAG-3Ј and reverse primer 5Ј-GCAGGGTCAAAACGTTGTGAC-3Ј). PCR reactions were performed in a 100-l volume of the ExTAKARA buffer containing 2.5 units of ExTAKARA Taq polymerase, 0.5 m each of the forward and reverse primers, 0.2 mm dNTP, and 5 l of genomic DNA. PCR products from D. simulans, D. yakuba, and some D. melanogaster lines were cloned using the TA cloning kit (Invitrogen, San Diego) as a means of checking the sequences obtained directly as PCR products. DNA sequencing was done with an ABI model 377 autosequencer using the Big Dye Terminator ready reaction kit according to the manufacturer’s protocol (Perkin-Elmer, Norwalk, CT).
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