Can epigenetic control explain pronounced within-plant heterogeneity of meiosis in a translocation trisome of Secale L.?

Introduction Detailed reports on variation in meiotic behaviour within individual organisms are rare. Sybenga et al. (2008) reported on the variation between tillers of single plants of rye (Secale cereale [...]
Meiotic metaphase I configuration frequencies were determined in different tillers of genetically related plants of rye (Secale cereale L.) heterozygous for reciprocal translocation T248W (between chromosome arms 1RS and 6RS) and with an additional (telocentric) arm 1RS. Seventeen different configurations could be recognized, grouped into three categories. Very different configuration frequencies were found not only between sister plants from the same parents but also between tillers of the same plant grown under identical conditions (climate chambers at 15°C and 20°C). The heterogeneity reflects variation in chromosome pairing and crossing over, and is variable and unpredictable. Anthers within florets were homogeneous. Between tiller heterogeneity is insufficient to explain differences between sister plants. It is ascribed to random somatic variation in the conditions of the chromatin which, at meiosis, govern chromosome pairing. During sexual differentiation, these conditions are fixed and subsequent cell lineages have the same pairing and crossing over characteristics. As homology search is an activity of DNA, this control of pairing and crossing over, consistent over long cell lineages, may be considered to be epigenetic even when no realistic suggestions concerning its character can be given. Key words: translocation heterozygote, single-arm-trisomiy, Secale, meiosis, intraplant heterogeneity. Les frequences des configurations observees lors de la metaphase I de la meiose ont ete mesurees au sein de differents talles de chacun de plusieurs plants de seigle (Secale cereale L.) genetiquement apparentes et heterozygotes pour la translocation reciproque T248W (entre bras chromosomiques 1RS et 5RS) ainsi que porteurs d'un bras (telocentrique) 1RS additionnel. Dix-sept configurations differentes ont ete distinguees et formaient trois categories. Des frequences tres differentes de ces configurations ont ete observees non seulement entre plantes soeurs ayant les memes parents, mais aussi entre talles de la meme plante cultivee dans des conditions identiques (chambres de croissance a 15°C ou 20°C). Cette heterogeneite reflete de la variation au niveau de l' appariement chromosomique et des enjambements, est variable et imprevisible. Les antheres au sein d' une fleur etaient homogenes. L' heterogeneite observee entre talles etait insuffisante pour expliquer les differences entre plantes soeurs. Ces dernieres sont plutot attribuees a une variation somatique aleatoire dans l' etat de la chromatine qui, lors de la meiose, determinerait l' appariement. Au cours de la differenciation sexuelle, ces conditions seraient fixees et les lignees cellulaires derivees partageraient les memes caracteristiques en matiere d'appariement et de recombinaison. Comme la recherche d'homologie est un processus qui depend de l'ADN, on peut considerer ce controle de l' appariement et de l' enjambent, conserve tout au long de lignees cellulaires, comme etant un processus epigenetique bien qu'aucune hypothese realiste concernant sa nature ne puisse etre mise de l'avant. Mots-cles : heterozygote pour une translocation, trisomie pour un seul bras, Secale, meiose, heterogeneite intra- plante. [Traduit par la Redaction]