Some pecan varieties have near 100% overlap of pollen production and pistillate flower receptivity and are capable of pollinating themselves. There is something unusual about the genetics of varieties that overlap. Let’s find out what.
Pecan dichotomy is determined by two alleles, ‘P’ = Protogynous and ‘p’ = Protandrous. Capital ‘P’ indicates that Protogynous is dominant and lower case ‘p’ indicates that Protandrous is recessive. You might attempt to do a standard Mendelian grid and expect native pecan to segregate with 1 - ‘PP’, 2 - ‘Pp’, and 1 - ‘pp’ which would be the norm with most genes. But this is not what happens with pecan. Because these alleles affect the bloom period and because they tend to force pistillate flowers to be pollinated by the opposite genotype, the result is that a Protandrous plant most of the time pollinates a Protogynous flower and vice versa. The result is that standard segregation patterns do not apply. In native stands of pecan, trees tend to segregate 50% protandrous and 50% protogynous. This means that about half of the time, native trees will have genotype ‘Pp’ and the other half ‘pp’.
But there are exceptions. One of the most notorious is Mahan which has genotype “PP”. All of Mahan’s progeny are Protogynous. What about trees that have genotype “pp”? Well, they have a quirk or two too. A plant with this genotype will always have pollen shed either prior to or concurrent with receptive female flowers. So what did I figure out that is unusual? Pecans that have near 100% overlap between pollen shed and flower receptivity are always genotype “pp”. There are no examples that I have found so far where a tree with ‘PP’ or ‘Pp’ genotype have strong overlap. Here are some examples: Barton, Cherokee, Creek, Jubilee, San Saba, and Western Schley. What makes this interesting? Most protandrous pecans do not overlap male and female flowers, therefore there must be a gene(s) that affects flowering type to cause them to overlap. I’m going to speculate that this will be a gene or biopath linked with chilling hour requirements.
Why so? A study of chilling hour requirements for pecan suggests that catkin buds have different chilling hour requirements to break dormancy than pistillate buds. I speculate that there is a gene(s) that sets both catkin and pistillate buds with the same chilling requirement so that they always break at the same time so long as chilling hour requirements are met. In other words, pollination type of pecan whether protandrous or protogynous is linked to the biopath for chilling hour requirements to break bud dormancy. The knock on effect is that there must be more than one ‘p’ allele but not necessarily more than one ‘P’ allele!
I’m posting a new update to the pollination chart tonight. Have fun! I am!