In Favor of Tetraploids

Most daylilies have either two sets of chromosomes, one set from each parent (diploid) or they have four sets of chromosomes, two sets from each parent (tetraploid). When hybridizing, diploid plants have certain advantages. It is important, however to realize that tetraploids may have particular advantages over diploids as well. The main disadvantage that tetraploids have when compared to diploids is the necessity of producing many more seedlings from each cross to attain the same diversity of offspring as in diploids. Tetraploids may also be more difficult to work with when it comes to producing viable seeds.

Tetraploids, in plants in general, tend to have larger nuclei and larger cells, they also seem to be somewhat hardier, larger plants with larger, and thicker flower parts. One way of looking at this might be that for every gene/enzyme, tetraploids have twice the amount of DNA responsible for the production of protein (enzymes and other proteins). As enzymes influence the speed of the chemical reactions, the higher the concentration of an enzyme the faster a particular reaction will occur (faster, in this case, means that the end products of the reaction will probably be in a higher concentration). The higher the concentration of end products the greater the effect on the phenotype. This is why one might expect the flowers to be larger and thicker and it might be the reason the flowers are more ruffled. With partially dominant genes for pigment production (if they occur), doubling the number of chromosomes may produce higher concentrations of pigment producing enzymes thus producing a more intense color. Diploid plants could have only two pigment producing alleles while tetraploids could have up to four pigment producing alleles. Characteristics influenced by genes which show incomplete dominance will be influenced by having four genes producing enzymes much more than those having only two genes.

When tetraploid daylilies first became popular, they did so because of the larger flowers and other characteristics. There was little if any genetic reason to make crosses with the hopes of coming up with brand new characteristics. True, characteristics showing partial dominance such as flower size, thickness, ruffled edges and a number of other characteristics, were important to hybridizers but these were only differences in degree. The prime example, of course, is the ruffled edge on some of the newer daylilies. Some diploids showed a tendency towards ruffling but it might be theorized the added DNA of the tetraploids would increase the enzyme/protein concentration. In partially dominant characters, the higher the concentration of enzyme, the greater the phenotypic effect, in this case the ruffling of the edge. At first, brand new characteristics were rarely found.

Through time, one would expect that gene and chromosomal mutations would occur in the DNA of tetraploid daylilies. Assuming that they are occurring at the same rate as in diploids and tetraploids contain twice the DNA of diploids, we might expect tetraploids to have twice the mutations as diploid plants. The large majority of these mutations will be recessive and will not show up in the phenotype until many crosses with itself have been made. The few dominant or partially dominant mutations will normally occur in very small numbers. However, one might expect dominant mutations to occur more often in tetraploid plants than in diploid plants because there are more alleles present.

Chromosomal mutations involving the transfer of genes from one place in a chromosome to another or from one chromosome to another may be somewhat more common in tetraploids than in diploids because there are more chromosomes involved. This may allow for genes to be "turned on" at different times and therefore have somewhat different effects from normal. Possibly a gene influencing color in the eye might be changed in its location and then affect the color of the throat, edge or self. This increases the possible variation in the plants.

With characteristics demonstrating partial dominance, the phenotype or appearance may change with each additional gene producing an enzyme. With two genes (diploid) there may be a noticeable characteristic and with four genes (tetraploid) producing the enzyme the characteristic will be much more noticeable. How many of you breeders have tried doubling the chromosomes of tetraploid plants? If this is successful, the resulting octoploid (4 x 2 = 8) plants should show particular characteristics in an extreme condition. Possibly there will be increased flower thickness, pigmentation, ruffling, teeth and other characteristics. The increase in ruffling could be such that the flower would rarely open. Along with this might be decreased fertility. While there might, very well, be problems, the attempt would be worth it, if nothing else, just to determine which genes are dominant and which are show partial dominance or multiple gene inheritance.