Evaluation of the cost of restoration of male fertility in Brassica napus

Gynodioecy frequently results from the interplay of mitochondrial cytoplasmic male sterility (CMS) and nuclear fertility-restoration genes. Models suggest that maintaining cytonuclear gynodioecy requires that restorer genes incur a cost to fitness because otherwise they would increase toward fixation. Direct tests of costs of restorer alleles require knowledge of the underlying genetics of sex determination. We use a well characterized CMS system in Brassica napus L. to measure aspects of fitness in four lineages that vary in whether they carry the pol CMS gene or male-fertile cytoplasm (cam), and whether they carry the Rfp restorer of pol or Rfn restorer of the nap CMS gene. As expected, plants with pol CMS and only the Rfn restorer experienced reduced flower size, stamen length, and pollen counts. Plants with pol and the Rfp restorer showed incomplete restoration with shorter stamens than both lines with cam cytoplasm and reduced pollen counts compared with plants with cam cytoplasm and the Rfp restorer. Among plants with cam cytoplasm, pollen counts were higher for those with the Rfp than Rfn restorer, indicating a greater cost of restoration associated with Rfn. These results demonstrate that costs of restoration differ for the Rfn and Rfp alleles in B. napus. Key words: Brassica napus, CMS, cost of restoration, cytoplasmic male sterility, gynodioecy. La gynodrecie resulte frequemment de l'influence reciproque des genes mitochondriaux controlant la sterilite male cytoplasmique (CMS) et des genes nucleaires qui restaurent la fertilite. Des modeles suggerent que le maintien de la gynodrecie cyto-nucleaire necessite que les genes de restauration assument un cout vers l'aptitude phenotypique car sinon ils augmenteraient vers la fixation. Des tests directs des couts des alleles de restauration necessitent que l'on connaisse la genetique qui sous-tend la determination sexuelle. Les auteurs ont utilise un systeme de CMS bien connu chez Brassica napus L. afin de mesurer les aspects de l'aptitude phenotypique chez quatre lignages qui varient sur le fait qu'ils possedent gene de CMS pol ou un cytoplasme male-fertile (cam), et qu'ils possedent Rfp, le restaurateur de pol, ou Rfn, le restaurateur du gene de CMS nap. Comme prevu, les plants possedant pol et le restaurateur Rfn seulement produisaient des fleurs dont la taille, les etamines et le decompte en pollen etaient reduits. Les plants possedant le CMS pol et le restaurateur Rfp presentaient un retablissement incomplet avec des etamines plus courtes que celles des deux lignages possedant un cytoplasme cam, et un decompte en pollen reduit comparativement aux plants possedant un cytoplasme cam et le restaurateur Rfp. Parmi les plants possedant un cytoplasme cam, les decomptes en pollen etaient plus eleves que ceux qui possedaient le restaurateur Rfp plutot que Rfn, indiquant qu'un cout de restauration superieur est associe a Rfn. Ces resultats demontrent que les couts de restauration different entre les alleles RJh et Rfp chez B. napus. [Traduit par la Redaction] Mots-cles: Brassica napus, CMS, cout de restauration, sterilite male cytoplasmique, gynodrecie.
Introduction Gynodioecy, in which populations contain both hermaphrodites and females, is the second most common angiosperm breeding system after hermaphroditism (Richards 1997; Sakai and Weller 1999). As implied by the [...]