Datura wrightii flower
Hi all, it has been some time since I have posted a story, I apologize. I have been traveling the country, visiting awesome plants and ending up in Tucson, AZ. I’m studying Datura wrightii flowers (seen at left) and their pollinators and herbivores with Dr. Judie Bronstein at the University of Arizona.
In any case, I last threatened to discuss the implications of having sex with yourself. Now, the answer may be intuitive. For example, humans normally do not wish to mate with close relatives because a) it seems abhorrent to us, and b) it probably seems abhorrent because the children of these relationships often have severe physical or mental problems.
Why does this happen? Let’s say there is a gene mutation out there that causes severe physical problems, and let’s say it is recessive, or that it is masked by the normal version of the gene when they are together in a cell (called the heterozygous state). Now this normally doesn’t pose a problem because of the masking effect. If, however, two people who are both heterozygous (likely if they are related to each other) mate, there is a 25% chance, on average, that their offspring will have both forms of the mutant gene (called the homozygous recessive state). Then, there is no masking at all, and the mutation causes the negative changes in that offspring.
The same thing happens in plants, except that they can actually have sex with themselves, and don’t need another separate plant. So, inbreeding in plants can cause serious problems in the offspring. ‘Like what?’ you might ask? Well, read on!
In a seminal work using our old friend Mimulus guttatus, Ivey and Carr (2005) manipulated plants so that one set was produced from inbreeding parents and one set from non-inbreeding, or outcrossing parents. They then measured important floral traits like the diameter of the petals (together they are called the corolla), the length of the petals, and the distance between the stigma (the female bit) and the anthers (the male bits). They also counted how many flowers were produced and seed set. Finally, they counted how many pollinators visited the plants and how long each pollinator took to forage on the flower.
Figure showing effect of inbreeding on seed set
Not surprisingly, inbreeding significantly decreased corolla diameter, corolla length, the number of flowers produced, and the number of seeds produced (see figure above). This might be expected if mutant genes are being expressed in the homozygous state through inbreeding. Perhaps through the effects on corolla size, inbred plants also received fewer initial pollinator visits
The implications of these results are that once inbreeding starts, plants may get fewer pollinator visits and thus may start to self-pollinate, leading to even more inbreeding. One could imagine that this cycle could continue until the population goes extinct or the population evolves to be completely self-pollinating, which we sometimes see in nature. It might be more likely that a few outcross pollen grains landing and fertilizing eggs in the population bring enough genetic diversity to avoid such cycles. A little genetic diversity can go a long way!
With that, I’ll sign off — I hope everyone has a bloomin’ Fourth of July!
Ivey, C. T. and D. E. Carr. 2005. Effects of herbivory and inbreeding on the pollinators and mating system of Mimulus guttatus (Phrymaceae).American Journal of Botany 92: 1641-1649