| dc.description.abstract | Nearly two thousand diploid hybrid plants originating
from crosses between triploids (with two sets of Italian ryegrass
chromosomes and one set of perennial ryegrass chromosomes) and
diploid Italian ryegrass were clonally replicated and examined
for their expression of five qualitative and five quantitative
traits in a glasshouse. The qualitative trait were: presence
of awns, red leaf base and three isozyme variants; phosphogluco
isomerase (PGI), glutamate oxaloacetate transminase (GOT) and
superoxide dismutase (SOD). The quantitative traits were; leaf
width, leaf length, per cent stem, regrowth score and days to ear
emergence.
The purpose of using triploids during backcrossing was to
reduce the proportion of genes originating from the donor parent.
Assuming normal mendelian inheritance, the progeny would have 1/6
or 17% of the genes of perennial ryegrass origin compared to 1/4
or 25% in two generations of ordinary backcrossing: only a small
advantage in using triploids. But loss of chromosomes of the
donor species due to preferential pairing at meiosis in the triploid
would greatly increase that advantage. The occurrence of such
chromosome loss was best revealed by the frequency of hybrid plants
with qualitative traits. The frequency of plants with the
perennial ryegrass allele at the PGI/2 isozyme locus and with red
base was much lower than expected but the frequency of plants with
the perennial ryegrass allele at the GOT locus was exactly as
expected. Since these three loci were unlinked, the results
suggest considerable loss of some chromosomes but not others.
There was a clear linkage between leaf length and red
base, PGI and GOT suggesting that genes for leaf length were
distributed widely over the genome. Genetic correlation between
quantitative traits suggested linkage between early flowering
and stemmy regrowth, both traits of Italian ryegrass.
Potentially useful hybrid clones which showed only a
single perennial ryegrass trait were identified. Some had no
awns, some with red base, some had entirely leafy regrowth and
some were late flowering. However, because some of the
quantitative traits had high coefficients of variation and
because the traits examined only represent a small proportion of
the ryegrass genome, further work is required to determine the
value of these clones for breeding. | en |
