Evolutionary History of Plants - Mechanisms and Players in Evolution of Plant Form

Mechanisms and Players in Evolution of Plant Form

While environmental factors are significantly responsible for evolutionary change, they act merely as agents for natural selection. Change is inherently brought about via phenomena at the genetic level - mutations, chromosomal rearrangements and epigenetic changes. While the general types of mutations hold true across the living world, in plants, some other mechanisms have been implicated as highly significant.

Genome doubling is a relatively common occurrence in plant evolution and results in polyploidy, which is consequently a common feature in plants. It is believed that at least half (and probably all) plants have seen genome doubling in their history. Genome doubling entails gene duplication, thus generating functional redundancy in most genes. The duplicated genes may attain new function, either by changes in expression pattern or changes in activity. Polyploidy and gene duplication are believed to be among the most powerful forces in evolution of plant form; though it is not known why genome doubling is such a frequent process in plants. One probable reason is the production of large amounts of secondary metabolites in plant cells. Some of them might interfere in the normal process of chromosomal segregation, causing genome duplication.

In recent times, plants have been shown to possess significant microRNA families, which are conserved across many plant lineages. In comparison to animals, while the number of plant miRNA families are lesser than animals, the size of each family is much larger. The miRNA genes are also much more spread out in the genome than those in animals, where they are more clustered. It has been proposed that these miRNA families have expanded by duplications of chromosomal regions. Many miRNA genes involved in regulation of plant development have been found to be quite conserved between plants studied.

Domestication of plants like maize, rice, barley, wheat etc. has also been a significant driving force in their evolution. Some studies have tried to look at the origins of the maize plant and it turns out that maize is a domesticated derivative of a wild plant from Mexico called teosinte. Teosinte belongs to the genus Zea, just as maize, but bears very small inflorescence, 5-10 hard cobs and a highly branched and spread out stem.

Interestingly, crosses between a particular teosinte variety and maize yields fertile offspring that are intermediate in phenotype between maize and teosinte. QTL analysis has also revealed some loci that, when mutated in maize, yield a teosinte-like stem or teosinte-like cobs. Molecular clock analysis of these genes estimates their origins to some 9,000 years ago, well in accordance with other records of maize domestication. It is believed that a small group of farmers must have selected some maize-like natural mutant of teosinte some 9,000 years ago in Mexico, and subjected it to continuous selection to yield the familiar maize plant of today.

Another interesting case is that of cauliflower. The edible cauliflower is a domesticated version of the wild plant Brassica oleracea, which does not possess the dense undifferentiated inflorescence, called the curd, that cauliflower possesses.

Cauliflower possesses a single mutation in a gene called CAL, controlling meristem differentiation into inflorescence. This causes the cells at the floral meristem to gain an undifferentiated identity and, instead of growing into a flower, they grow into a lump of undifferentiated cells. This mutation has been selected through domestication since at least the Greek empire.