Quantitatively determined self-incompatibility : 1. Theoretical considerations

Why are there so few homomorphic multi-locus sporophytically determined self-incompatibility systems?

Homomorphic multi-locus sporophytically determined self-incompatibility systems are much rarer than multi-locus gametophytic systems. This note examines some of the possible reasons for this disparity and concludes that, while each additional locus in a gametophytic system allows increased crossing among related plants as well as a lower mutation rate to maintain a given level of variability, the same conclusion cannot be drawn for sporophytic systems.

Quantitatively determined self-incompatibility. IV. Pollination and seed set in Borago officinalis

The outcomes of sequential double pollination, mixed double pollination and single pollination are compared. Single pollination leads to lower seed set than double pollination. Systematic differences between female genotypes are shown to be possible. It is also shown that failure to set seed is generally due to pre-zygotic maternal factors.

Quantitatively determined self-incompatibility : 3. Genetical variability in Borago officinalis

It is shown by simulation that a hypothetical multilocus, quantitatively determined self-incompatibility system, whether gametophytic or sporophytic, should maintain variability in small populations at a higher level than would panmixia. Studies of more than 20 isozyme loci show that borage has almost no variability.

Quantitatively determined self-incompatibility : 2. Outcrossing inBorago officinaux

It has been claimed that Borage (Borago officInalis L.) has a multifactorial self-incompatibility system. Such systems may have a high level of ineffective pollination, and we show that this is the case in borage. The ranking of seed set from highest to lowest is as follows: bee-pollination; natural pollination in the absence of bees; artificial cross-pollination between unrelated plants; artificial cross-pollination between related plants; artificial self-pollination. In diallel crosses, significant parental effects were detected but no consistent patterns of seed set, which suggest a simple self-incompatibility system, were detected. The level of outcrossing with natural pollination was very variable but greater than 50%. Thus, there appears to be no straightforward self-incompatibility system in borage.