Summary and analysis of the article: Evolution of pollen to ovule ratios and breading system in Erodium( Geraniaceae)
For this blog I read an article on the evolution of pollen to ovule ratios and breading systems in Erodium (Alarcon, 2011). Erodium is a genus in the Geranium family and has an interesting pattern of differential reproductive strategies
. The article distinguishes two main groups of species within the genus: perennial species (living more than one year) and annual species (living for one growing season). Most of the perennial species have, in addition to a longer life span, a medium to high pollen to ovule ratio, dichogamous (gametes mature at different times on hermaphroditic flowers) and herkogamous (gametes are separated spatially from each other on hermaphroditic flowers, see photo to left) strategies, and are endemic to the Mediterranean. In contrast, annual species typically occupy disturbed sites, lack strong dichogamy or herkogamy, a low pollen to ovule production ratio, and a much wider distribution. The authors of the article attempted to answer three main questions: 1.) How did these different breeding systems evolve? 2.) Did the annual lifecycle and self-fertilization in the genus evolve together? 3.) How did geo-climate changes affected the evolution of the genus?
. The article distinguishes two main groups of species within the genus: perennial species (living more than one year) and annual species (living for one growing season). Most of the perennial species have, in addition to a longer life span, a medium to high pollen to ovule ratio, dichogamous (gametes mature at different times on hermaphroditic flowers) and herkogamous (gametes are separated spatially from each other on hermaphroditic flowers, see photo to left) strategies, and are endemic to the Mediterranean. In contrast, annual species typically occupy disturbed sites, lack strong dichogamy or herkogamy, a low pollen to ovule production ratio, and a much wider distribution. The authors of the article attempted to answer three main questions: 1.) How did these different breeding systems evolve? 2.) Did the annual lifecycle and self-fertilization in the genus evolve together? 3.) How did geo-climate changes affected the evolution of the genus? There are many different types of breeding systems in flowering plants. Because plants can have both male and female components on a single flower, different mechanisms have evolv
ed to either ensure self-pollination or out-crossing (See right for self-pollinating flower. Notice how the anthers surround the stigma). For example, a flower might have the male component mature more quickly than the female component. This type of strategy (known as dichogamy) ensures out-crossing because the male and female components on the same flower are fertile at different times. Differences in the breeding systems of flowers have been described as methods of reducing or improving the likelihood of self-reproduction and/ or improving the efficiency of sex roles (Burd et al. 2009).
ed to either ensure self-pollination or out-crossing (See right for self-pollinating flower. Notice how the anthers surround the stigma). For example, a flower might have the male component mature more quickly than the female component. This type of strategy (known as dichogamy) ensures out-crossing because the male and female components on the same flower are fertile at different times. Differences in the breeding systems of flowers have been described as methods of reducing or improving the likelihood of self-reproduction and/ or improving the efficiency of sex roles (Burd et al. 2009). As a general rule, the amount of pollen produced per ovules produced per flower is similar to the idea of clutch size in animals. Each gamete produced has a resource cost associated with it as well as a benefit as long it contributes to a fertilized zygote. But, depending on the variability of pollination success, it may be more beneficial to produce more pollen per ovule in some cases, and less in others.
The basic distinction between annual and perennial species (the amount of time to life cycle completion) shows that both r and k selection pressures are present. It may be more beneficial for plants present in a rapidly changing environment to produce more offspring over a shorter period of time (annual, r-selection). However, if the environment is not changing rapidly, a plant may live many years and only reproduce when optimal (perennial, k-selection). Another study I looked at shows that a strong r-selection on annual flowering plants in general gives more benefit to self-fertilization than out-crossing (Snell, and Aarssen, 2005).
In the article I read, the amount of self pollination was found based on the amount of viable seeds produced when the flowers were enclosed in pollen-proof bag. In addition, the amount of pollen produced per ovule per flower was counted. The authors of this article found that in Erodium more pollen produced per ovule is typically a sign that the species is 
focusing on out-crossing, or mixed mating (outcrossing sometimes and self-fertilizing other times), whereas a low pollen to ovule ratio is something to be expected from obligate self-fertilizing species (This makes sense if you think about it because if fertilization doesn’t require a pollinator, much less pollen is wasted; and therefore less is needed). Using a phylogeny of the genus, in conjunction with breeding systems and pollen/ovule ratios, they found that an annual self-fertilizing trend was present in basal members of the genus before members of the species had settled in the Mediterranean (see phylogeny to the left; the key in the top left corner refers to the pollen/ovule ratios). This is an important discovery because many scientists had hypothesized that self-fertilization was an “evolutionary dead-end”, according to Alarcon. Although self-fertilization has its consequences (see F and H text on inbreeding depression), it also has several important benefits. Because a pollinator isn’t needed, self-fertilizing plants can colonize barren land (where no pollinator would have interest in visiting). Once the area is settled, the plants could diversify through mutations preventing self-fertilization. It appears that this was the case in Erodium. The self-fertilization strategy was present in ancestral species and then was reduced or lost in some lineages. This could be accomplished by increased dichogamy or herkogamy.
focusing on out-crossing, or mixed mating (outcrossing sometimes and self-fertilizing other times), whereas a low pollen to ovule ratio is something to be expected from obligate self-fertilizing species (This makes sense if you think about it because if fertilization doesn’t require a pollinator, much less pollen is wasted; and therefore less is needed). Using a phylogeny of the genus, in conjunction with breeding systems and pollen/ovule ratios, they found that an annual self-fertilizing trend was present in basal members of the genus before members of the species had settled in the Mediterranean (see phylogeny to the left; the key in the top left corner refers to the pollen/ovule ratios). This is an important discovery because many scientists had hypothesized that self-fertilization was an “evolutionary dead-end”, according to Alarcon. Although self-fertilization has its consequences (see F and H text on inbreeding depression), it also has several important benefits. Because a pollinator isn’t needed, self-fertilizing plants can colonize barren land (where no pollinator would have interest in visiting). Once the area is settled, the plants could diversify through mutations preventing self-fertilization. It appears that this was the case in Erodium. The self-fertilization strategy was present in ancestral species and then was reduced or lost in some lineages. This could be accomplished by increased dichogamy or herkogamy. Through observing pollinator visits per species, Alarcon found that species which competed for pollinators have larger flowers and more separation of male and female sexual organs. In areas where pollination events were rare, species had smaller flowers, and more overlap between mature male and female sexual organs. The latter strategy is hypothesized by Alarcon to be due to a shifting fitness peak: outcrossing is beneficial when pollinators are present; self-fertilization is beneficial when pollinators are absent. Slight changes in the degree of herkogamy and dichogamy may have allowed species to adapt to the presence or absence of pollinators during periods of climate change in different areas. Towards the end of the article, Alarcon mentions that the phenomenon of switching between self-fertilization and out-crossing was common amongst flowers at the time (possibly for the same reason of changing fitness peaks).
In summary, this article gives a classic example of multiple and shifting fitness peaks. These peaks occur based on r and k selection pressures. The r selection peak favors fast reproducing individuals with short life spans. Whereas the k selection peak favors optimal reproduction and a longer life span. These two fitness peaks shifted depending on available pollinators in a time of variable seasons and climate. If there were no pollinators, then individuals with pollen-efficient small flowers, as well as less herkogamy and dichogamy, left more offspring than other members of the population. If there were many pollinators, then individuals with the largest/showiest flowers with ambient pollen production could compete for pollinators. These individuals could also reduce the amount of self-fertilization by increasing the degree of herkogamy and dichogamy. If the pollinators were unreliable, then a good strategy would be to have self-fertilization occur later in life (which has been observed in mixed-mating members of the genus). This strategy gives the plant a chance to outcross in one part of its life, while keeping self-fertilization as an option. Self-fertilizing individuals have the advantage of assured pollination, and therefore may be able to colonize disturbed lands. The ability to easily reduce the amount of self pollination once the pollination community becomes established appears to have been a major key to the success and prevalence of the genus.
Sources:
Alarcon, Maria L., C. Roquet, J. Aldasoro. 2011. Evolution of pollen/ovule ratio and breeding system in Erodium (Geraniaceae). Systematic Botany 36: 661-676. http://www.bioone.org/doi/full/10.1600/036364411X583637
Burd, M., T. L. Ashman, D. R. Campbell, M. R. Dudash, M. O. Johnston, T. M. Knight, S. J. Mazer, R. J. Mitchell, J. A. Steets, and J. C. Vamosi. 2009. Ovule number per flower in a world of unpredictable pollination. American Journal of Botany 96: 1159–1167.
Snell, R. and L. W. Aarssen. 2005. Life history traits in selfing versus outcrossing annuals: exploring the ‘time-limitation’ hypothesis for the fitness benefit of self-pollination. BMC Ecology 5: 2.
Web Resources:
More information on plant breeding systems: http://www.sciencedirect.com/science/article/pii/S0960982206019178
Website of a cool botanist (check out the publications in the evolution and sexual systems categories)
This article gave an interesting plant-based example of mixed mating strategies, that mirrors the animal examples proposed in class. The the case of the Erodium lineages, there were out crossing, selfing and even mixed strategy species. These were showed to correlate to pollen production and energy allocation to reproduction. The study showed that the ancestral trait was for self-fertile plants; and that only some of the lineages from the Mediterranean had out crossing strategies. This may be confounding if self-fertile plants are considered evolutionary dead-ends. But even with selfing, there is some (not as much) recombination of alleles added onto the higher chance of producing an offspring in an area where pollination could prove difficult would result in a favorable trait.
ReplyDeleteIt could also be mentioned that some plant species reproduce entirely asexually. Such as the colonies of all-male Elodea (water weed) found as a weed in our area, was thought to originate from a introduction from an aquarium. In the relatively stable aquatic ecosystem this plant can be highly successful reproducing entirely asexually.
Overall, the plant kingdom has some very interesting examples of varying life strategies for breeding. From microscopic orchid seeds that require a fugal symbiont to germinate to the massive seed of a coconut that floats from island to island complete with a generous meal for the developing embryo.