Plant Rarity - Plant of the Week (month? summer?) #2 Hybridization

I wanted to write about a few aspects of my research from school, specifically hybridization/polyploidy and population genetics . Here's a start on the hybridization stuff.

A couple of papers came out in the early 1990's by Norman Ellstrand, telling a cautionary tale that it was possible that rare plants could be hybridized out of existence by a process called "genetic swamping". Swamping occurs when the rare plant is outnumbered by a nearby closely related plant, and the volume of heterospecific pollen results in fewer and fewer pure offspring of the rare species. One of the best known cases of swamping is in the Catalina Island Mahogany (that CB and I got at least sort of close to when we went whale watching), Cercocarpus traskiae. It occurs on a specific soil type (on an island, no less), and the current population estimate is that there are 7 mature individuals and 20-50 immatures. Another species of Cercocarpus, C. betuloides var. blancheae, is also present on the island and studies have shown the two species hybridize. So much so that the original work by Rieseberg et al. (1989) suggested that there were only two pure C. traskiae individuals and what they thought were five more were actually hybrids.

This is a recent status report that has more details, but the short version is that C. traskiae is in a holding pattern after workers removed the more common species and fenced off the area against herbivores.

I looked at two species of Physaria, both endemic to Colorado. The rarer of the two is P. bellii, which occurs in three counties, and the more common is P. vitulifera, which occurs in six. That's a P. bellii photo up at the top, notice the rounded leaves. The ranges of the two overlap a little, and reports of purported hybrids in that area got me interested in the project in the first place.

But wait, there's more. This is a wierd thing about Physaria, but one species can have populations that are different ploidy levels. The genus is sort of wacky to begin with because of their generally low numbers of chromosomes. For example, in some populations of P. vitulifera, all members are diploid, with 8 chromosomes. Other populations are tetraploid, and have 16 chromosomes. They are all P. vitulifera, but the tetraploid ones have an extra set of chromosomes. It isn't known how the offspring do when a diploid individual crosses with a tetraploid, thereby making a triploid individual with 12 chromosomes. Generally, though, triploidy isn't a good thing, because it leaves some chromosomes without partners when it's time for meiosis. Below is a shot of P. bellii root tip cells, with 8 chromosomes.

So one of the things I investigated was the ploidy levels of a sample of P. vitulifera populations that were near P. bellii populations. The hypothesis was that if nearby populations of P. vitulifera were tetraploid, they wouldn't be a threat to P. bellii, which is always diploid and has 8 chromosomes. I was extremely lucky that there were so few chromosomes to work with, because I still did hundreds of squashes to get enough data to make some generalizations. The populations of P. vitulifera near P. bellii were generally tetraploid, except those that occurred close to the hybrid population, suggesting that hybridization between the species would likely be confined to known sites, and therefore P. bellii wasn't threatened by genetic swamping.

Above is P. vitulifera in the rocky soil that it's found in. Notice the leaves that are kind of violin shaped.

OK, enough for one post. I'll cover the population genetics stuff next time.

C. traskiae pic from here.

Ellstrand, NC. (1992) Gene flow by pollen: Implications for plant conservation genetics. Oikos 63: 77-86.

Ellstrand NC, Elam DR (1993) Population genetic consequences of small population size: implications for plant conservation. Annu Rev Ecol Syst 24: 217-242.