Thursday, August 9, 2007

PS 62-97: Mating system in a tropical pine with contrasting population sizes

Rafael F. Del Castillo and Sonia Trujillo Argueta. Instituto Politecnico Nacional

The number of adult individuals in populations of Pinus chiapensis, a tropical pine, spans five orders of magnitude, and seed viability decreases significantly as population size decreases.  The smallest populations, composed of less than 50 individuals, had very low seed viability, and no evidence of natural regeneration. We estimated the mating systems in populations contrasting in size using isozymes.  Since inbreeding depression probably biased upwards the outcrossing rates, we obtained an estimate of inbreeding depression at earlier stages of the life cycle based on the regression of per-family F values, obtained at seed stage, with germination rates, and use such an estimate to correct our secondary mating systems estimations. We found that multilocus outcrossing rates (tm) varied widely among populations.  In two populations tm was not significantly different than one, the largest studied population had mixed mating system (tm =.5), and in the smallest studied population tm was not significantly different than zero.  We concluded that in small populations, increasing probabilities of selfing rates and other sources of inbreeding are likely involved in the observed decrease in seed viability and lack of regeneration through inbreeding depression.

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