Triplet repeat variability in the signal peptide sequence of the Xmrk receptor tyrosine kinase gene in Xiphophorus fish

Trinucleotide repeats in several human genes have been found to undergo spo ntaneous variation in repeat numbers in succeeding generations. Expansion o f the repeat beyond a certain length causes specific pathological disorders . So far, a naturally occurring triplet repeat instability of transcribed s equences has been reported only from humans. However, the signal peptide en coding region of the receptor tyrosine kinase gene Xmrk from fish of the ge nus Xiphophorus contains a CTG repeat that differs in length even between c losely related individuals. The consequence of this variability is signal p eptides with shorter or longer hydrophobic core regions reaching, in some i ndividuals, the critical maximum length for functional protein export or ev en exceeding it. In one stock, animals that are homozygous for such an alle le were extremely rare, indicating that the triplet repeat length variabili ty of the Xmrk gene of Xiphophorus may indeed have an influence on the func tion of the gene product and, under certain conditions, may affect the fitn ess of the individual. (C) 1998 Elsevier Science B.V. All rights reserved.