Ask Mr. Smarty Plants

This is a very interesting questiion.   As it turns out, about 95% of all flowering plants are "sexually monomorphic" meaning that they have both male and female parts on the same plant.   The may be hermaphroditic (about 90%) with the male and female parts in the same flower (perfect flower) or they may be monoecious (about 5%) with separate male and female flowers but with both flowers on the same plant.  Then there are the dioecious plants (about 5%) having separate male and female flowers with male flowers on one plant and female flowers on a different plant.  The situation with  Vitis rotundifolia (Muscadine) is that it is can be dioecious with male and female flowers on different plants, but it can also be hermaphroditic with the male and female parts in the same flower (perfect flowers) on the same plant. 

Determination of sex in Vitis is controlled by a single gene (with perhaps influence from other genes). The gene that determines the sex of the plant has 3 different forms—called alleles.  The 3 different alleles are:  M = male, H = hermaphrodite, and F = female.  Any plant can have only two of these at once—one from pollen and one from the egg.  The combination that results in the seed inside the fruit determines the sex of the plant that grows from the seed.   As it turns out M, H and F are not equal.    The hierarchy is M>H>F.   This means that plants that have both alleles the same will show the sex of that allele, i. e.:  MM = male flowers on the plant; HH = perfect flowers with both male and female parts on the plant; FF = female flowers on the plant.  The surprising result is what you get when you combine different alleles.   Since M>H>F,  MH and MF will produce a plant with only male flowers and HF produces hermaphroditic plants.   The only way there can be plants with only female flowers is with the combination of FF.  So:

• male-flowered plants can be:  MM, MH and MF
• hermaphroditic plants can be HH and HF
• female plants can only be FF 

Keep in mind that male plants produce pollen but don't have eggs to be pollinated and thus can't produce fruit.  Female plants have eggs to be pollinated but no pollen.  So—male plants can't cross with male plants and female plants can't cross with female plants.  However, males and females can cross and hermaprodites can cross with either males or females, with other hermaphrodites or can self-cross. 

Here are the results for all possible crosses:

MM (male)  X  FF (female) = MF (male)

MH (male)  X  FF (female) = MF (male), HF (hermaphrodite)

MF (male)  X  FF (female) = MF (male), FF (female)

MM (male)  X  HH (hermaphrodite) = MH (male)

MH (male)  X  HH (hermaphrodite) = MH (male), HH (hermaphrodite)

MF (male)  X  HH (hermaphrodite) = MH (male), HF (hermaphrodite)

MM (male)  X  HF (hermaphrodite) = MH (male), MF (male)

MH (male)  X  HF (hermaphrodite) = MH (male), MF (male), HH (hermaphrodite), HF (hermaphrodite)

MF (male)  X  HF (hermaphrodite) = MH (male), MF (male), HF (hermaphrodite), FF (female)

HH (hermaphrodite)  X  FF (female) = HF (hermaphrodite)

HH (hermaphrodite)  X  HH (hermaphrodite) = HH (hermaphrodite)

HH (hermaphrodite)  X  HF (hermaphrodite) = HH (hermaphrodite), HF (hermaphrodite)

HF (hermaphrodite)  X  FF (female) = Hf (hermaphrodite), FF (female)

HF (hermaphrodite)  X  HF (hermaphrodite) = HH (hermaphrodite), HF (hermaphrodite), FF (female)

All of this goes to show that in order to get a male plant, you need to have pollen from male plant (either MM, MH or MF) with the M allele.  According to the USDA Agricultural Research paper, "Pollination Biology of the Muscadine Grape, Vitis Rotundifolia Michx." by Blair Sampson et al.,  Vitis rotundifolia (Muscadine) is pollinated both by insects and by wind.   The answer, then, to how your male-flowered vine appeared is that probably one of the flowers on your female plants or on your perfect-flowered hermaphrodite plants was pollinated by insects or by the wind carrying pollen with the M allele from a male plant with the genotype MM, MH, or MF growing in your area.  The resulting fruit with its seed carrying the M allele then grew into your vine.  Alternatively, some animal ate and deposited under your steps a seed from a grape (from a female or hermaphrodite plant) that was the result of pollen from a male-flowered plant.

If you want to know more about sex determination in grapes and you have access to academic-type journals at a university nearby, here are a couple of papers you might like to read:

Dalbo, M. A., G. N. Ye, N. F. Weeden, H. Steinkellner, K. M. Sefc and B. I. Reisch.  2000.  A gene controlling sex in grapevines placed on a molecular marker-based genetic map.  Genome 43:333-340.

Marguerit, E., C. Boury, A. Manicki, M. Donnart, G. Butterlin, A. Nemorin, S. Wiedemann-Merdinoglu, D. Merdinoglu, N. Ollat, and S. Decroocq.  2009.  Genetic dissection of sex determinism, inflorescence morphology and downy mildew resistance in grapevine.  Theoretical and Applied Genetics 118:1261-1278.