| Trofolium Repens L. - more commonly known as | | | | plants heterozygous at either locus in fact exert |
| white clover - can release hydrogen cyanide (HCN) in | | | | partial dominance for the amount of HCN produced. |
| a process called cyanogenesis. This inherited trait is | | | | Plants with Ac Ac, Li Li are shown to produce |
| determined by two unlinked complementary genes, | | | | approximately twice as much HCN, while one |
| resulting in populations which contain both cyanogenic | | | | functional allele at each locus (Ac ac, Li li) creating an |
| and acyanogenic plants. The study of cyanogenic | | | | intermediary dosage effect. A homozygous recessive |
| white clover presents an excellent opportunity to | | | | pair of alleles at either loci will mask the phenotypic |
| observe many basic genetic characteristics including | | | | effect of any dominant allele demonstrating epistasis |
| independent segregation, gene interaction, epistasis | | | | and produce distorted F2 Mendelian ratios of 9:7. |
| and variable expressivity. More importantly, its natural | | | | White clover also shows individual variable |
| polymorphism and allotetraploid status (i.e. having four | | | | expressivity. Plants which have the same cyanogenic |
| genomic copies, 2n=4x=32) has allowed for valuable | | | | genotype can express different phenotypes at |
| genetic and ecological research into population | | | | different times and in different parts of the plant (Till, |
| variance and environmental adaption, not least | | | | 1986). Environmental factors such as temperature |
| because its diploid inheritance suggests divergence of | | | | and altitude also seem to have an effect on the |
| ancestral species. | | | | amount of cyanoglucoside biosynthesis. Perhaps the |
| The two independently segregating loci that code for | | | | most important question from an ecological |
| the biosynthesis of HCN are designated as follows: | | | | perspective relates to why such polymorphic |
| Ac/ac, which codes for the presence/absence of | | | | distribution of white clover exists. What forces of |
| either linamarin and lotaustralin cyanoglucosides; and Li | | | | selection are at work? Much of the evidence shows |
| li, which codes for the presence/absence of the | | | | that the cyanogenic trait does serve as a defence |
| catabolic enzyme linamerase. A simple model of the | | | | mechanism against predators, with some species of |
| biochemical pathway follows a two-step process with | | | | slug selectively feeding on acyanogenic strains. |
| both the glucoside substrate and hydrolysing enzyme | | | | Herbivory has to be considered economically by |
| needed to successfuly produce HCN. Cyanogenic | | | | agriculturalists. On the contrary, some species of |
| plants will only release HCN when the plant tissue is | | | | moth and insect selectively feed on cyanogenic |
| ruptured and the two substances come into contact: | | | | strains. If the cyanogenic strain offers protection to |
| in the intact plant they are usually separated. | | | | the plant, why does the acyanogenic strain prevail? |
| Corkhill (1942) was the first person to formally | | | | Some studies suggest that a balance exists because |
| categorise the four types of white clover. Test | | | | of a trade off between traits needed for survival - |
| crosses of homozygous plants at each allele led him | | | | this is supported by evidence of increased fitness of |
| to propose a dominant mode of inheritance for | | | | acac plants compared to Acac plants. |
| cyanogenesis. Subsequent research has shown that | | | | |