Hayes and Garber - Cucurbit Breeding

MODE OF REPRODUCTION IN RELATION TO BREEDING 47
From these and other results (Jones, 1918) it is apparent that
selfing in maize produces:
1. Strains which can not be perpetuated.
2. Strains which can be perpetuated only with difficulty.
3. Strains which exhibit normal development but vary
growth attained.
in amount of
EXPLANATION OF HYBRID VIGOR 1
The studies of the early hybridizers, Koelreuter, Gartner,
Knight, and others, gave results which can be summed up in a
single sentence as follows (East and Hayes, 1912)
:
" Crosses between varieties or between species often give hybrids with
a greater vegetative vigor than is
possessed by either parent."
Darwin made extended and careful studies of the effects of
cross- and self-fertilization in plants. He conclusively proved
that in general there is an advantage in cross-fertilization. While
he noted some self-fertilized families he believed these would
eventually perish. Lacking as he did a knowledge of Mendelian
phenomena it was impossible for Darwin to develop as logical
an explanation of these results as we now have. Darwin thought
the results could best be explained by the nature of the sexual
elements rather than in the act of crossing.
Several explanations of hybrid vigor have been advanced
since the rediscovery of Mendel's law. In all cases heterozygosis
has received a major place in the explanation.
The results of
these studies have been summed up as follows (East and Hayes,
1912):
"1. The decrease in vigor due to inbreeding naturally cross-fertilized
species and the increase in vigor due to crossing naturally self-fertilized
species are manifestations of one phenomenon. This phenomenon is
heterozygosis. Crossing produces heterozygosis in all characters by
which the parent plants differ. Inbreeding tends to produce homozygosis
automatically.
"2. The phenomenon exists and is in fact widespread in the vegetable
kingdom.
"3. Inbreeding is not injurious in itself, but weak types kept in
1
A recent monograph by EAST and JONES (1919) presents in a clear and
concise way the effects of inbreeding and cross-breeding in the light of
modern theories of genetics. This publication has been used very freely
in this section.