Friday, 3 August 2012

Primula’s little secret

“The earthly manifestations of God’s world began with the realm of plants, as a kind of direct communication from it. (…) Plants were bound for good or ill to their places. They expressed not only the beauty but also the thoughts of God’s world”
Carl Jung

I have heard from many people that plants are boring creatures because of their incapacity of movement, but on the other hand this is an incredible challenge they have to deal with all the time. They have to answer to with certain problems even though they cannot move or see to, for example, choose the visitors of their flowers. They need to be selective enough to attract pollinators, avoiding nectar robbers, herbivores and their own pollen (self-pollination); they are to me one of the most incredible examples of the mysterious abilities of Nature. There are many fascinating examples on pollination strategies to avoid inbreeding in plants and Primula seems to be quite talented on finding the right strategy on this matter, avoiding selfing like no other. 

Primula acaulis. Source: Flora-on

To avoid selfing, Primula flowers managed to find a way to create hermaphrodite flowers of two different types, each type can only pollinate the other type, and each individual only forms one of the morphotypes. Pin plants (morphotype 1) have a long style and short anthers; thrum plants (morphotype 2) have a short style and long anthers. Pollen of pin plants can only grow down on the style of thrum plants and the pollen of thrum plants only fertilizes pin plants. For this reason, flowers of one type cannot be fertilized by pollen of their own type, and since one individual can’t produce both types inbreeding is hardly possible to happen. 

Morphotype 1 cannot fertilize morphotype 2 and vice-versa, as the pollen is only "fertile" with the opposite morphotype

The secret recipe for this special strategy can be easily explained genetically – pin plants are heterozygous (Aa) or homozygous-dominant (AA) and thrum plants are homozygous-recessive (aa). Because crossing happens only between both morphotypes, there are only two possibilities of crossing, Aa x aa or AA x aa. 

Crossing between both morphotypes can be explained in two ways, depending on the genotype of the pin plants. If they are homozygous (AA, example on the left), all offspring will also be of the pin morphotype because their genotype will certainly be heterozygous (Aa)

However, it is not as simple as this, as there is not only one locus responsible for this character, but a group of three loci tightly linked together (linkage), forming a complex locus or a super gene. Because these genes are so closed together, a cross-over that separates them is highly improbable to happen, however it may happen in rare situations and only in this rare situations self-pollination might occur. When crossing-over takes place within those loci, two complementary kinds of homostyle are formed, with the stigma characteristic of one type and the pollen characteristic of the other, allowing self-fertilization to happen. These homostyles are though naturally eliminated from most populations because of the selective disadvantage that inbreeding brings along. 

Primula veris thrum mophotype (aa genotype). Source: Flickr (padikeo)

Primula veris pin morphotype (Aa or AA genotype). Souce: Flickr (Viveka)

So now, whenever you find a Primula flower, take a look at the reproductive structures, and you will be able to tell if they have an AA/Aa genotype or an aa genotype!