User:Decasg/Kin selection

Kin Selection in Plants
Though originally thought unique to the animal kingdom, evidence of kin selection has been identified in the plant kingdom.

Competition for resources between developing zygotes in plant ovaries increases when seeds are sired by different fathers. How developing zygotes differentiate between full siblings and half-siblings in the ovary is undetermined, but genetic interactions are thought to play a role. Nonetheless, competition between zygotes in the ovary is detrimental to the reproductive success of the mother plant, as fewer zygotes mature into seeds, and is also thought to harm the mother plant itself. As such, the reproductive traits and behaviors of plants suggests the evolution of behaviors and characteristics that increase the genetic relatedness of fertilized eggs in the plant ovary, thereby fostering kin selection and cooperation among the seeds as they develop. These traits differ among plant species. Some species have evolved to have fewer ovules per ovary, commonly one ovule per ovary, thereby decreasing the chance of developing multiple, differently fathered seeds within the same ovary. Multi-ovulated plants have developed mechanisms that increase the chances of all ovules within the ovary being fathered by the same parent. Such mechanisms include dispersal of pollen in aggregated packets and closure of the stigmatic lobes after pollen is introduced. The aggregated pollen packet releases pollen gametes in the ovary, thereby increasing likelihood that all ovules are fertilized by pollen from the same parent. Likewise, the closure of the ovary pore prevents entry of new pollen. Other multi-ovulated plants have evolved mechanisms that mimic the evolutionary adaption of single-ovulated ovaries; the ovules are fertilized by pollen from different individuals, but the mother ovary then selectively aborts fertilized ovules, either at the zygotic or embryonic stage.

After seeds are dispersed, kin recognition and cooperation affects root formation in developing plants. Studies have found that the total root mass developed by Ipomoea hederacea (morning glory shrubs) grown next to kin is significantly smaller than those grown next to non-kin ; shrubs grown next to kin thus allocate less energy and resources to growing the larger root systems needed for competitive growth. Interestingly, when seedlings were grown in individual pots placed next to kin or non-kin relatives, no difference in root growth was observed. This indicates that kin recognition occurs via signals received by the roots.

Groups of I. hederacea plants also display greater variation in height when grown with kin than when grown with non-kin. The evolutionary benefit provided by this was further investigated by researchers at the Université de Montpellier. They found that the alternating heights seen in kin-grouped crops allowed for optimal light availability to all plants in the group; shorter plants next to taller plants had access to more light than those surrounded by plants of similar height.

The above examples illustrate the effect of kin selection in the equitable allocation of light, nutrients, and water. The evolutionary emergence of single-ovulated ovaries in plants has eliminated the need for a developing seed to compete for nutrients, thus increasing its chance of survival and germination. Likewise, the fathering of all ovules in multi-ovulated ovaries by one father, decreases the likelihood of competition between developing seeds, thereby also increasing the seeds' chances of survival and germination. The decreased root growth in plants grown with kin increases the amount of energy available for reproduction; plants grown with kin produced more seeds than those grown with non-kin. Similarly, the increase in light made available by alternating heights in groups of related plants is associated with higher fecundity.

Kin selection has also been observed in plant responses to herbivory. In an experiment done by Richard Karban et al, leaves of potted Artemisia tridentata (sagebrushes) were clipped with scissors to simulate herbivory. The gaseous volatiles emitted by the clipped leaves were captured in a plastic bag. When these volatiles were transferred to leaves of a closely related sagebrush, the recipient experienced lower levels of herbivory than those that had been exposed to volatiles released by non-kin plants. Sagebrushes do not uniformly emit the same volatiles in response to herbivory: the chemical ratios and composition of emitted volatiles vary from one sagebrush to another. Closely related sagebrushes emit similar volatiles, and the similarities decrease as relatedness decreases. This suggests that the composition of volatile gasses plays a role in kin selection among plants. Volatiles from a distantly related plant are less likely to induce a protective response against herbivory in a neighboring plant, than volatiles from a closely related plant. This fosters kin selection, as the volatiles emitted by a plant will activate the herbivorous defense response in related plants only, thus increasing their chance of survival and reproduction.

Mechanisms of Kin Selection in Plants
The ability to differentiate between kin and non-kin is not necessary for kin selection in many animals. However, because plants do not reliably germinate in close proximity to kin, it is thought that, within the plant kingdom, kin recognition is especially important for kin selection. Unfortunately, the mechanisms by which kin recognition occurs in plants remain unknown. Below are some hypothesized processes involved in kin recognition.


 * Communication through Roots: Plants are thought to recognize kin through the secretion and reception of root exudates . It has been hypothesized that exudates actively secreted by roots of a plant are detected by roots of neighboring plants  . However the root receptors responsible for recognition of kin exudates, and the pathway induced by receptor activation, remain unknown . The mycorrhizae associated with roots might facilitate reception of exudates secreted by neighboring plants, but the mechanism through which this may occurs is also unknown.
 * Communication through Volatiles: In Karban et al's study of kin recognition in Artemisia tridentata (described in the section above), the volatile-donating sagebrushes were kept in individual pots, separate from the plants that received the volatiles . This suggests that root signaling is either not involved in, or not necessary to induce a protective response against herbivory in neighboring kin plants . Karban et al hypothesize that plants may be able to differentiate between kin and non-kin based on the composition of volatiles emitted by neighboring plants . Because the donor volatiles were only exposed to the recipient sagebrush's leaves, it is likely the volatiles activate a receptor protein in the plant's leaves. The identity of this receptor, and the signaling pathway triggered by its activation, both remain to be discovered