Androdioecy is a sexual system in which males co-occur with hermaphrodites, which have both male and female function. Stable androdioecy is rare in nature, and theory suggests that it requires that males sire more than twice as many offspring as hermaphrodites. In several members of the olive family (Oleaceae), androdioecy occurs with higher frequencies of males than predicted by theory. In Phillyrea angustifolia L., we found that high male frequencies can be maintained in natural populations because hermaphrodites belong exclusively to one of two self-incompatibility groups, and thus, each can fertilize only half of all pollen recipients. In contrast, males can pollinate all hermaphrodites. Thus, in this species, the reproductive disadvantage that males face due to the loss of female function is offset by the fact that all males are fully compatible with all pollen recipients.

Tlic reproductive capacities of the females (male-steriles or mS) of l h y m u~ uuigarii L. ha\(. Iwcti i t ivestigated, in terms of size and weight of plants, seed production, germination ability and pollination. The data, collected during nine years under controlled conditions and in natul-al situations are discussed in comparison with the results known in the liter-aturr. It is suggestcd that the large diversity of characteristics promoting sexual polymorphistn could justifv the high and variable rates of tnS met in natural conditions in T. vulgaris.

Nucleocytoplasmic determination of male sterility in Thymus vulgaris L. has been assumed in all papers attempting to explain the remarkably high frequencies of male steriles found in natural populations of this species. This paper provides strong evidence that both nuclear and cytoplasmic genes are involved in the determination of male sterility of this species, giving a complex inheritance. Interpopulation and intrapopulation crosses have shown that the ratio of females versus hermaphrodites among offsprings varied widely from 1∶0 to 1∶1. Furthermore, interpopulation crosses consistently yielded a higher frequency of females than intrapopulation crosses. Nucleocytoplasmic inheritance was demonstrated by an absence of male fertiles in backcrosses and asymmetrical segregation in reciprocal crosses. Molecular analysis of the mitochondrial DNA of some of the parents used in crosses suggested the involvement of different cytoplasms in the inheritance of male steriliy.

SummaryA rare homomorphic diallelic self-incompatibility (DSI) system discovered in Phillyrea angustifolia (family Oleaceae, subtribe Oleinae) can promote the transition from hermaphroditism to androdioecy. If widespread and stable in Oleaceae, DSI may explain the exceptionally high rate of androdioecious species reported in this plant family. Here, we set out to determine whether DSI occurs in another Oleaceae lineage.We tested for DSI in subtribe Fraxininae, a lineage that diverged from subtribe Oleinae c. 40 million yr ago. We explored the compatibility relationships in Fraxinus ornus using 81 hermaphrodites and 25 males from one natural stand and two naturalized populations using intra-and interspecific stigma tests performed on F. ornus and P. angustifolia testers.We uncovered a DSI system with hermaphrodites belonging to one of two selfincompatibility (SI) groups and males compatible with both groups, making for a truly androdioecious reproductive system. The two human-founded populations contained only one of the two SI groups.Our results provide evidence for the evolutionary persistence of DSI. We discuss how its stability over time may have affected transitions to other sexual systems, such as dioecy.