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As early as 1942, a rather complex self-incompatibility mechanism in sugar beet was discovered by Owen, but his two-locus interpretation appeared to be insufficient.

Observations presented in this paper, including F1, F2, F3, F4 and F2 × F3 families, all favoured a model involving four gametophytic S-loci with complementary interaction. It is pointed out that more than the four S-loci so far identified may exist in a hidden state of homozygosity.

This complementary S-mechanism is in principle identical with the two-locus system in the monocotyledonous family Gramineae. Caryophyllales, the taxonomic order of Beta, is generally considered a rather close derivative of those early dicotyledones that also gave rise to the monocotyledones. Hence it is suggested that these early dicots possessed several co-operating S-loci, and that multi-S-locus systems may still exist in many species, even among those hitherto considered to carry only one S-locus. Apparently, many researchers did not seriously realize the possibility of multi-locus systems, and therefore worked with too limited materials, methods and hypotheses.