FREQUENCY CHANGES IN MIXTURES OF Af-af, St-st, Tl-tl GROWN UNDER MINIMAL
AND INTENSE COMPETITION.
Glover, T. J. Hobart & Wm. Smith Colleges, Geneva, NY USA
A fundamental principle in population biology is the concept of fitness.
The fitness or realized reproduction of an individual depends on the organism's
phenotype interacting with the environment. A particular genotype may show
a wide range of fitness under different physical conditions and competitive
situations. Because organisms exhibit this flexibility of response with
respect to fitness it is unlikely that a single genotype of plant raised in
monoculture will produce the maximum number of seeds (offspring) under the
varying pressures encountered in field conditions.
In a plant such as Pisum where the commercial yield is closely correlated
with a major component of fitness, namely total number of seeds produced,
investigations of changes in fitness may have important practical consequences.
As a preliminary study of fitness and competition in Pisum, mixtures
of eight different genotypes affecting foliage type were tested under two
forms of spatial competition in the field at Geneva, NY. The eight genotypes,
furnished by G. A. Marx, were isogenic lines derived as F6's from a backcross
involving three major gene loci.
The genotypes utilized were all possible combinations of Af-af, St-st,
and Tl-tl. Afila homozygotes have leaflets replaced by extra tendrils, st
homozygotes have reduced stipules, and tl (acacia) homozygotes have tendrils
replaced by leaflets. The interaction between af and tl produces clusters of
small leaflets. No other obvious morphological changes occur due to inter-
action in any of the eight genotypes.
The eight genotypes were mixed together in equal ratios and grown on
trellises in the field in 1977. Phenotypes were scored in the seedling stage.
Dry seed was bulk-harvested and replanted on trellises in the field in 1979.
Again plants were scored as seedlings and dry seed was harvested. Samples
from 1979 seed were grown to seedling stage in the greenhouse and scored.
The ratios of the phenotypes of these seedlings represent the population ratios
after two generations of field selection under minimum spatial competition.
A similar planting scheme was followed with a mixture of the eight
genotypes grown under intense spatial competition in drilled plots. These
conditions were comparable with commercial cultural conditions. The drilled
plot initial mixture of seed inadvertently contained twice as much of the
AfAf stst T1T1 genotype as the other genotypes.1/ This error may have con-
founded the results due to density dependent selection. However, for ease
of comparison such selection was assumed to be negligible and the AfAf stst
TlTl were halved as correction procedure.
The results (Table 1) show dramatically the changes in composition of
the mixtures. The equal numbers of phenotypes in the field both on trellises
and in drilled plots in 1977 indicate equal and high germination rates for
all genotypes. After one generation the mixtures from trellises and drilled
plots were significantly different from the initial ratios and from each other.
While fitness measured as total number of dry seeds is not perfectly correlated
with marketable yield, mixtures of two or more genotypes may increase yield.
