Wide nucleo-cytoplasmic polymorphism for male sterility in Origanum vulgare L.

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Abstract
Natural populations of O. vulgare contain 1–62 percent male-sterile (mS) plants. The inheritance of male sterility in P1, V2, and V3 families involved a distinct cytoplasmic factor interacting with specific restorer genes (Rf1, Rf2, and Rf3) in each case. The inheritance patterns of 21 additional mS plants, collected from diverse populations, are reported here. Each mS plant was crossed with four male testers (all heterozygous for Rf1, two for Rf2, and two for Rf3). Nine mS plants gave segregation patterns like P1, none like V2, and two like V3 cytoplasms. The remaining mS plants were characterized by at least three segregation patterns in sharp contrast with those of P1, V2, and V3 cytoplasms. Four mS plants (V4, V5, V6, and Y7) gave only male-fertile (mF) progenies with the male testers, within-family studies demonstrated monogenic inheritance in each family. However, reciprocal between-family crosses showed that the genes acting in each family represent distinct loci (designated respectively Rf4 to Rf7)- Distinct asymmetric results were obtained in some reciprocal crosses (e.g., in Y7 vs. other families; V5 vs. V6, P1 vs. other families, and in V5 vs. V6) suggesting cytoplasmic differences between such families. In studies of seven families (P1, V2 to V6 and Y7] where reciprocal between-family crosses were accomplished, results indicated different restorer genes and probably distinct cytoplasmic interactions in every case. Five of these families came from the same population (V2 to V6). Other mS plants of this population gave different results as did the mS plants tested from other populations. The same natural population seems therefore to contain a large number of nucleo-cytoplasmic factors for male sterility. The biological significance of such large cytoplasmic polymorphism in natural populations and its potential application in plant genetics and plant breeding are discussed.