Rock pink rocks! A tale of two sexes

Rock pinks, members of a species known as Dianthus sylvestris, are long-lived perennial herbs that are rather common throughout the Alps. The species is gynomonoecious-gynodioecious, and now that you know about plant sexes, you translate this rather tedious jargon into the following greeny sexy sex taxinomy: this species has female-, hermaphroditic individuals, and “undecided” (gynomonoecious) plants bearing both female- (pistillate) and hermaphroditic (perfect) flowers.

In this post, we’ll investigate whether it is best considering sex at the flower level (we have here only pistillate or perfect flowers) or at the plant level (we have three kinds of plants). In other words, is mating influenced by the plant’s sex in this species?

First, as illustrated on the left, pistillate flowers are smaller than perfect ones.

Second, pistillate flowers cannot self-fertilize, since they lack stamens.

Third is trickier. Perfect flowers have both female and male organs (pistil and stamens), and could self. But the species evolved simple characteristics to prevent selfing, and each sex is expressed differentially in the flower timelife: stamen mature first and pollen is released before the pistil is mature. That is, the flower begins its life as male, and end up as a female. In the botanical jargon, this is known as protandrous dichogamy. Protandrous means maleness comes first, dichogamous means the two sex phases differ in time.

But the species is still self-compatible, which means that if you apply pollen grains from the male stage onto stigma of the same flower when it turns female, you will obtain a fruit (in this species, a capsule) full of seeds. That is, dichogamy evolved to prevent selfing, though evolving auto-incompatibility would have been much more efficient. The reason is that hermaphroditic individuals can still self through geitonogamy, i.e. when several flowers are open simultaneously. You just need a pollinator passing by, visiting a flower in a male stage and then a flower in a female stage, on the same plant.

A fairly common exercise in plant biology is to investigate outcrossing or selfing rates in your favorite species. With a little bit of simple genetics, you can tease apart offspring that are born of self-pollination events (your mom is also your dad, so that your genotype is constituted uniquely by mom’s alleles), and offspring that are born of outcrossing (mom and dad are different individuals, so that your genotype contain some alleles that are different from mom’s ones). You can then calculate a ratio of offspring that are resulting from either mating event. Please keep in mind that stigma receive pollen grains from multiple donors, so that a fruit is often constituted by half-sibs in plants. Some of which result from selfing.

This is what was done in the study* presented here (figure below). On the Y-axis, you have outcrossing rates (ranging from 0, i.e. complete selfing, to 1, i.e. complete outcrossing). Outcrossing rates were averaged by flower type by plant: pistillate flowers on female plants (FF), perfect flowers on pure hermaphrodites (HH), and pistillate or perfect flowers on mixed plants (FM and HM respectively). On the X-axis, you have the average size of each flower category.

So here are the results:

1- Pistillate flowers (FF) are highly outcrossing. This is no wonder since such flowers need pollen grains from other plants. (the only way they could have apparent selfing based on genetics is if they mate with a relative, a phenomenon called bi-parental inbreeding).

2- Perfect flowers from pure hermaphrodites (HH) have a selfing rate of about 20% (1 offspring every 5 results from self-fertilization). This gives an idea of the pollen flow within plants due to simultaneous open flowers.

And here are very interesting facts:

3- Pistillate flowers on mixed (gynomonoecious) plants (FM) have a selfing rate similar to that of flowers from pure hermaphrodites (ca 20%), while

4- perfect flowers on mixed plants (HM) have a higher outcrossing rate than their hermaphroditic colleagues on pure hermaphrodites (average selfing rate is 10% instead of 20%).

How is that possible?

Well, there’s a fairly simple explanation. That’s all about pollinators discrimination of flower size. Pink rock flower size is globally similar at the plant level, though they may differ between individuals (indeed, female plants have much smaller flowers compared to hermaphrodites). But on mixed plants, difference in size due to flower sex is highly contrasted within individuals. And guess what? Pollinators prefer bigger flowers. That is, when they visit a mixed plant with both flower types open simultaneously, they’ll visit perfect flowers first (because of their size), and pistillate flowers next. It means that perfect flowers on these plants will receive more pollen from the previously visited plant (an outcrossing event), while smaller pistillate flowers will receive more self-pollen (since the pollinators just visited a perfect flower from the same plant). And there we are!

So sex is not just a matter of flower kinds. It is also impacted by the sexual phenotype at the plant level (see, I didn’t use “gender“). And amazingly, this is also an illustration that higher organization levels can influence processes occuring at lower organization levels…

 

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* Collin C. L and Shykoff J. A. (2003). Outcrossing rates in the gynomonoecious-gynodioecious species Dianthus sylvestris (caryophyllaceae). American Journal of Botany 90(4): 579–585.

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