Thursday, May 21, 2015

Gynodioecy and the Wild Strawberry - Fragaria virginiana - Fraisier sauvage

So, why will I talk about gynodioecy today? Well, I noticed that Fragaria virginiana (wild strawberry) is blooming! F. virginiana is a gynodioecious species, so I thought I might try my hand at explaining why this plant is of so much interest to scientists. Gynodioecious species like this one help us to investigate questions about the evolution of separate sexes (among other things). Definitions, explanations, and much more below, after my usual more basic commentary about this lovely species.

Fragaria virginiana in bloom
This species is very widely distributed, native to all parts of North America (range map here). It is not at risk anywhere in its US range [1] (and, in fact, is considered a weed in some parts of its US range for its ability to spread prodigiously [1]), and is secure in all parts of its Canadian range except Nunavut, where it is sensitive [2].

This plant produces a fruit which is edible (and delicious!), and is one of the two species which was used to breed Fragaria x ananassa (commercial strawberries) -- the other species being the Fragaria chiloensis (coastal strawberry) [3,4,5].

Fragaria virginiana flower
F. virginiana grows primarily in meadows and open spaces [3,5,6] and will also reproduce via vegetative propagation (not just sexual reproduction!) [3,5,7,8].

Now hold onto your hats, ladies and gentlemen, we're going to talk about the evolution of separate sexes!

In animals, dioecy (separate male and female individuals for sexual reproduction) is the norm. In plants, however, dioecy is relatively uncommon, with only roughly 6% of known species of angiosperm (flowering plants) having separate sexes [9]. The primordial state for angiosperms (flowering plants) is monoecy, where both male and female gametes are produced by any given individual (hermaphroditism). So this raises the question of what evolutionary path has led from hermaphroditism to dioecy.

There are a number of proposed avenues for the evolution of dioecy. I'm only going to talk about one of them today: gynodioecy. This pathway is the most commonly studied [9], likely because gynodioecy is relatively common in plants, with about 7% of known species being gynodioecious [9].

In a gynodioecious species, there are two types of flower: female flowers, and hermaphroditic ("perfect") flowers. The categorization of the flowers as female or hermaphroditic is based more on function than appearance. This is because the occurrence of this type of sex distribution begins with a deactivation of male fertility in some individuals [9,10] -- in other words, some of the individuals stop producing pollen or stop producing viable pollen but may still, at first, have the physical structures associated with pollen production (there are a number of possible ways that this can occur, but for today's post just the fact that it happens should suffice). At this point, we have two types of flowers: those which are functionally female (minimal or no male reproduction -- nobody's getting pollen from them), and those which are functionally hermaphroditic (their pollen fertilizes others, and they receive pollen from others).

So it's fairly intuitively obvious that the hermaphroditic individuals have a reproductive advantage in this kind of scenario: they get more chances to pass on their genes. To make this easier to understand, I've created a little table which should help to illustrate the situation. Let's take the theoretical reproductive opportunities for any given individual. That individual, if it produces fruit, has guaranteed that it gets to be the female parent of those seeds; and if it produces pollen, has the chance of also being the male parent of other seeds. The columns represent the two parents of any given fruit (female parent and male parent).

Table 1. Reproductive opportunities (parentage) for hermaphroditic plants
Table 2. Reproductive opportunities (parentage) for female plants
Basically, out of 4 reproductive opportunities, the hermaphrodite takes 2. 2/4 certainly isn't bad! Flowers that don't produce pollen, however, lose their chance to be the male parent of other fruit, as illustrated in Table 2. So these flowers only take 1/4 opportunities to pass on their genes.

So wait, you might say. Wouldn't that mean that plants that are female will be outpaced by hermaphrodites, and their genes will just eventually disappear?

That's would certainly be the case if it weren't for female compensation. Those plants which aren't producing pollen anymore see a rise in seed viability [9,10]. Basically, their female fertility is increased when their male fertility ends. This is probably because the energy that was originally being devoted to producing pollen can be redirected toward improving seed viability [9]. So the female individual actually is also coming out better in this system; she gets a better return on her investment for the fruit and seeds she produces, producing more viable offspring from her fruit than a hermaphrodite.

From this point on we get to theory, but the principle is that because there is less competition for male parentage (more chances to fertilize a female flower), and because it takes a lot of energy to produce fruit and seed, it will be more and more advantageous for the remaining hermaphrodite flowers to shift toward investing more energy in pollen and less energy in fruit [9,10]. This process would eventually lead to dioecy (separate sexes).

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