Cabbages and kin:
According to someone whom I believe, the Emperor Diocletian was one of the most able administrators the Roman Empire ever had.  (Sorry about what he did to Christians.)  When he took the empire over, it was a shambles.  He split it east from west and appointed people he trusted to various components.  Then having everything humming along nicely, he retired. 

Things fell apart.  A delegation came to him and asked him to come out of retirement and straighten it all out.  He replied, “If you could see the cabbages I have been raising you wouldn’t even ask such a thing.” The Brassica genus, of which ordinary cabbage is a member, includes mustard seed, oilseed rape, swedes, kohlrabi (I think that one has been questioned recently.) turnips, cabbage, brussels spouts, cauliflower and broccoli, to the dismay of little boys everywhere.  The genetics (and epigenetics) of the genus continues to fascinate. (How Plants Avoid Incest, Daphne Goring and Emily Indriolo NATURE vol. 466 no. 7390 August 19, 2010 page 926 and Trans-acting Small RNA Determines Dominance relationships in Brassica Self-Incompatibility, Yoshiaki Taruntani, Hiroshi Shiba, Megumi Iwano, Tomohiro Kakizaki, Go Suzuki, Masao Watanabe, Akira Isogai and Seiji Takayama page 983 of the same issue)

As I have mentioned before, the ability of animals somehow to recognize kin and have lower fertility when mated with non kin is supported by a vast amount of evidence, while the evidence that plants do the same is thin.  But they ought to.

What has been found, is that Brassica can recognize and reject pollen from near related others, and this recognition and rejection are mediated by a mechanism that includes small non-coding RNA chains and also includes methylation. 

Well if they can recognize pollen that is too kin, it is possible that they can recognize pollen that is not kin enough.  There are a number of Brassica hybrids, so the process cannot be absolute.  It doesn’t have to be.  Since it is pollen that is being rejected, this amounts to a pre-zygotic infertility.  There could be a post-zygotic element as well.

So I await, although I do not hold my breath, the discovery of the converse of the inbreeding avoidance mechanism that has been studied. 

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