Pollination in Pecans

Pecans are wind-pollinated.  Trees are monoecious, meaning both male and female flowers are on the same tree. Thus, self-pollination is possible, but ultimately undesirable.  There are two main pecan flowering types: protandrous and protogynous.  When male flowers release their pollen before female flowers are receptive, those flowers are protandrous (also called Type I).  When female flowers are receptive to pollen before pollen is shed from the male flowers on the same tree it is called a protogynous flower or Type II.

Some protandrous (Type I) pecan trees include Caddo, Cape Fear, Cherokee, Cheyenne, Creek, Desirable, Gafford, Giles, Jackson, Oconee, Oklahoma, Pawnee, Peruque, and Western.  Some protogynous (Type II) pecan trees are Burkett, Candy, Choctaw, Elliot, Forkert, Kanza, Kiowa, Mahan, Maramec, Mohawk, Mount, Oakla, Podsednik, Schley, Shoshoni, Stuart, Sumner, and Wichita.

Pecan has what is called dichogamous flowering, when male and female flowers mature at different times.  Dichogamy promotes cross-pollination within and between species.  It is also known as heterodichogamy.  Dichogamy may be an intermediary step between synchronous dichogamy and dioecy (male and female flowers on separate plants).  The degree of dichogamy is variable within pecan trees and can be affected by weather.  Moist, warm springs favor male flowers, whereas cool, dry springs favor female flowers.  Some members of the hickory family may switch flowering type depending on the environment in the year.  This has been documented in Shagbark (Carya ovata) and Mockernut (Carya tomentosa) hickories. Complete dichogamy means that no self-pollination is possible.  Incomplete dichogamy results in some level of selfing.  Dichogamy encourages cross pollination and discourages self-pollination.  Self-pollination can lead to inbreeding depression in some plants.

Inbreeding depression occurs when two closely related individuals mate.  Some species have a strong negative response to this situation, whereas some have an intermediate response, and some little to none.  There can be ramifications of selfing, including fruit abortion, suppressed kernel development, and low plant vigor.  The unfit do not survive, thus resulting in a reduction in the number of successful mating individuals within a population.  Selfing also limits the gene flow from other populations.  Genes from other populations help to perpetuate individuals that adapt to environmental stresses.  Species with a strong tendency against self-pollination have greater genetic diversity within populations.  Within pecans, the level of inbreeding is low or inbred seedlings die early and never enter the mating process.

Thompson and Romberg (1985) reported that a single gene determines dichogamy in pecan trees.  This means that the trait is qualitative, or is controlled by a single gene or very few genes.  They reported that protogyny is the dominant trait and protandry is recessive.  This is common throughout the hickory family.  Beedanagari et al. (2005) found that protogyny and green stigmas were linked traits as were protandy and red stigmas.  These traits were tightly linked with little recombination.  This means that the more tightly linked the traits are, the rarer the recombination possibility will be.

There are benefits to dichogamy, with the largest being genetic variation.  More genetic variation leads to better pecan tree survival, continued evolution, and better climate adaptation.  Inbreeding, mating of close relatives or selfing, tends to bring out bad traits and thus ultimately makes the tree non-competitive with its non-inbred neighbors.

References

Beedanagari, S.R., S.K. Dove, B.W. Wood, and P.J. Conner. 2005.  A first linkage map of pecan cultivars based on RAPD and AFLP markers. Theoretical and Applied Genetics 110:1127-1137.

Thompson, T.E. and L.D. Romberg. 1985. Inheritance of heterodichogamy in pecan. Journal of Heredity 76:456-458.

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