Ca. Kimbeng et Et. Bingham, Population improvement in lucerne (Medicago sativa L.): components of inbreeding depression are different in original and improved populations, AUST J EX A, 38(8), 1998, pp. 831-836
Inbreeding depression, the lowered fitness of inbred individuals compared w
ith their non-inbred counterparts, is an important concept in lucerne impro
vement; but is poorly understood. Two-allele autotetraploid populations are
suitable for studying inbreeding depression, especially when the populatio
n improvement strategy involves inbreeding, because they are derived from c
hromosome-doubling of hybrid diploid plants. They have a maximum of 2 allel
es and a single allelic interaction per locus. Inbreeding depression was co
mpared in original 2-allele autotetraploid populations and populations that
had undergone inbreeding and selection. The original and improved (selecte
d) populations were produced by intercrossing 2 single-cross lines from the
original and improved 2-allele autotetraploid populations respectively. He
rbage yield of the S-1 and intercrossed generations derived from these popu
lations was evaluated in field trials at Arlington, Wisconsin, USA, and use
d to estimate inbreeding depression. Herbage yield of the S1 and intercross
ed generations derived from the improved population were significantly (P<0
.01) higher, by 13.3 and 24%, respectively, than those derived from the ori
ginal population. Selection during inbreeding probably decreased the freque
ncy of deleterious alleles and accumulated favourable alleles. Inbreeding d
epression values were higher in the improved compared with the original pop
ulation. Genetic load of deleterious alleles may account for much of the in
breeding depression observed in the original population, whereas, in the im
proved population,loss of heterozygosity or non-additive gene interactions:
between favourable alleles on linked chromosome segments may account for th
e substantial inbreeding depression. Therefore, in a population improvement
program, the causes of inbreeding depression seem to be more important tha
n their estimated value.