Husband BC. 2000. Constraints on polyploid evolution: a test of the minority cytotype exclusion principle. Proceedings: Biological Sciences 267: 217-223.
This author looked for assortative mating and frequency-dependent selection on tetraploids and diploids in laboratory fireweed populations. Fireweed occurs throughout North America, with populations of different ploidy usually geographically separated, though sympatric and parapatric populations occur in a zone of contact along the Rocky Mountains. In contrast to other plants, fireweed diploids occur more often at higher latitudes and at higher altitudes.
Frequency-dependent selection against rare genotypes (or cytotypes) is generally expected because in the absence of assortative mating, the rare type will suffer a greater proportion of failed matings (4x X 2x = 3x failure). Assortative mating driven by any of a large number of factors could therefore explain the apparent importance of polyploidy in evolution.
In contrast to expectations, tetraploid absolute fitness did not vary with frequency, but diploid absolute fitness did, leading to changes in relative fitness. While this specific result was unexpected, it is consistent with the general expectation of reduced relative fitness in rare types.
This author found that the threshold of frequency of equal relative fitness was approximately 0.61 tetraploid, suggesting a difference in absolute fitness favouring diploids. In this study, diploid plants had approximately twice as many flowers as tetraploids, and the tetraploid flowering period was about 10 days shorter (out of 53 days) than the diploid.
Two factors examined in this study could drive assortative mating among tetraploids, thus improving their odds of establishment (and “instant speciation”).
1. Tetraploid flowering time completely encompasses and extends beyond diploid. However, in this study, the opposite occurred.
2. Pollinator preferences drive a higher-than-expected frequency of 4 to 4 pollinator trips. When looking between individual plants, the opposite occurred; however, when visits between flowers within individual plants were considered, a very high proportion of trips were within tetraploid individuals, a potential source of assortative mating.
The author concludes that other, unexamined, factors may be driving assortative mating and polyploid evolution in this and other plants.
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4 comments:
"This author" may not read your post, but he is about 100ft down the hall from you.
Yes, I know. I hope I don't say anything here that could be percieved as disrespectful to Dr. Husband or anyone else. That is certainly not my intent. He is certainly more than welcome to read this blog and comment if he feels so inclined.
I've been using "this author" or "these authors" in almost all of my annotated bibliography entries for three reasons:
1. I frequently do not know the gender of the author or authors, so I don't feel comfortable saying "he did..." or "she found...".
2. I try to take a paper at only face value, and separate any feelings about authors (positive or negative) I may have. This way I hope to avoid missing good papers by authors who have previously written things I've disagreed with.
3. Some author names are difficult to spell or to type rapidly, particularly if the name includes accent marks. "This author" is fast enough for me to type that I don't interrupt my flow of thoughts when writing up my notes on a paper.
I'm just saying that if your goal is to enter into a discussion with the author, and he is on the same floor as you, you're better off walking down and talking with him personally.
you're better off walking down and talking with him personally.
Yes, I think I'll be taking advantage of the general friendliness of scientists (in my experience) when such opportunities arise. Thanks for the suggestion!
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