Population cytogenetics of Chorthippus vagans. II. Reduced meiotic transmission but increased fertilization by males possessing a supernumerary chromosome

Abstract

A submetacentric B chromosome was present at a low frequency (ca. 4%) in a natural population of the grasshopper Chorthippus vagans. Its heterochromatic nature is shown by its positive heteropycnosis during first meiotic prophase and by its response to C-banding. As its C-banding pattern and the meiotic autopairing of its two arms both demonstrate, it appears to have been derived from a metacentric iso-B chromosome by means of a pericentric inversion. The frequencies of equational division of the B univalents and the formation of abnormal gametes were significantly different between the 2 one B males found. A causal relationship is demonstrated between the equational division of the B at anaphase I and the formation of spermatid micronuclei, which can be assumed to contain B chromosomes that are being eliminated. Such B elimination was higher in male no. 2 than in male no. 37. The estimated B-transmission frequency was 0.28 for male no. 2 and 0.46 for male no. 37. The presence in the same population of polymorphisms for the M4 and M7 autosomes made it possible to determine the specific parents for four egg pods containing the B chromosome produced in a mass culture using wild-caught individuals. From this we calculate the actual transmission of the B by the one B males to be 0.27 for male no. 2 and 0.44 for male no. 37, which agrees with their respective estimated transmission frequencies. The relative fitness of zero B and one B males was calculated in terms of their viability and fertility components. While viability was lower for one B individuals, their fertility was much higher than that of zero B males. Indeed, sexual selection in favour of one B males was sufficiently high to counteract both the reduced transmission of the B by one B males and the phenotypic selection against one B zygotes. Key words: Chorthippus, meiotic drag, supernumerary chromosome, B chromosome, sexual selection.