daylily hybridization

daylily hybrid
daylily hybrid
daylily hybrid


Tetraploid plants are generally regarded as more desirable than their diploid counterparts. They can occur naturally, or by soaking seeds in a solution of colchicine, a toxin which interferes with microtubules within the cell and prevents chromosome separation during cell division. One cell ends up with double the normal number of chromosomes, while the other has no chromosomes and is doomed to die off.

Having two different sets of chromosomes per cell means that all the enzymes and protein products that allow every cell in the plant to function, have the potential to double in amount. Not everything doubles in amount, because (1) there are physical limits, (2) rate-limiting steps, (3) and cellular monitoring system to force some proteins to exist at a precise concentration. Enough things do double up to create thicker flowers, stockier stems, more disease resistance... depending of the plant species.

There are also unintended, undesirable effects. For instance, a thicker flower may be cause by thicker cellulose walls around each cell, opacifying the colored substance within each cells that impart the flower its colors. When tetraploid cymbidium orchids first occurred in cultivation, everyone was thrilled with the rounder, thicker and longer-lasting flowers; until it was realized that they were mostly whitish pastels and a lot of hybridizing work lay ahead to re-introduce the bold colors seen in species and diploid.

The genetics of tetraploid plants are more complex than for diploid flowers. The rate of phenotypic change is less than for diploid plants, and more progeny has to be planted in order to uncover recessive traits. Let's compare. Below is a standard F1 diploid cross for one trait:
ywhite Y ywhite Y ywhite
ywhite Y ywhite Y ywhite

Now, here is the same for a tetraploid plant:

ywhiteywhite YY ywhiteywhite YY ywhiteywhite
ywhiteywhite YY ywhiteywhite YY ywhiteywhite

In the F1 generation, both crosses are similar, all flowers are expected to be yellow. It gets complicated in the F2 generation.

Let's recall what the F2 diploid sibling cross looked like, with 25% of the offspring showing off the recessive white:

Y ywhite
Y Y Y Y ywhite
ywhite Y ywhite ywhite ywhite

Here is below the messy F2 tetraploid table, where only 6% of the flowers will show the recessive white:

YY Y ywhite Y ywhite ywhiteywhite
YY YYYY YYYywhite YYYywhite YY ywhiteywhite
Yywhite YYYywhite YYywhiteywhite YYywhiteywhite Yywhiteywhiteywhite
Y ywhite YYYywhite YYywhiteywhite YYywhiteywhite Yywhiteywhiteywhite
ywhiteywhite YYywhiteywhite Yywhiteywhiteywhite Yywhiteywhiteywhite ywhiteywhiteywhiteywhite

In practive, it means that it takes four times more effort to work in a recessive trait in a tetraploid breeding line than for a diploid one. Only 6% of the tetraploids, compared to 25% of the diploids.

E-mail Chantal.