James Harding

Quantitative studies on mating systems: I. Evidence for the non-randomness of outcrossing in Phaseolus lunatus

MATING systems are generally thought to be important factors in determining the amount and nature of genetic variability in populations. Many plant species have mating systems which allow partial inter-mating in addition to self-fertilisation; genetic models for such populations have been presented by Hayman and Mather (i), Hayman (i), Binet et al. (ig) and Workman and Allard (1962). Fyfe and Bailey (1951) have presented various procedures for estimating inbreeding coefficients,f, and outcrossing parameters, ; associated with loci scored, where = frequency of random outbreeding and (r —) = frequency of selfing (see also Nei and Syaktdo, 1958). Estimates of and (i —a) have been presented for naturally occurring populations of Datura metaloides by Snow and Dunford (1961). Similar results for an experimental population of Hordeum vulgare (Composite Cross V of C. A. Suneson) have been presented by Jam and Allard (1960). The purpose of this investigation is to study in detail the mating systems involved in certain artificial populations of Jima beans, Phaseolus lunatus. In all the previously mentioned models, outcrossing was assumed to be at random. In view of the results of Guitierrez and Sprague (1959), which showed rather conclusively that outcrossing was not at random in their maize populations, we shall attempt to investigate the randomness of observed outcrossing and possible trends in such cases where outcrossing appears non-random.

Rapid evolution of life history traits in populations of Poa annua L.

Two populations of Poa annua L., one from an irrigated area and one from a dry area, were studied to test the following hypothesis: different life cycles should be selected in habitats of contrasting mortality regime. Plants from the irrigated area exhibited a typical perennial habit (indeterminate growth and low reproductive effort) while those from the dry area showed traits characteristic of annuals (determinate growth, high reproductive effort and flowering synchrony) although total annuality was not achieved. The antagonism between growth and reproduction was expressed at the individual tiller level and was reflected in the life history traits of the populations. The characters studied exhibited a high broad sense heritability which is required for a rapid evolution of life cycles when selection pressures are strong. Different levels of constraints (ecological, evolutionary, physiological and developmental) have to be taken into account in order to understand this evolution.

Genetics of Lupinus : IV. Colonization and Genetic Variability in Lupinus succulentus.

SummaryPreliminary surveys indicate that most populations of Lupinus succulentus are genetically monomorphic for three flower color loci, viz., BB PP DD. In one small geographic area, a number of populations were polymorphic for the D/d locus. In this case, clinal variation and seasonal variation were found. The S/s locus, affecting seed coat pattern, was polymorphic in a large majority of populations and genetic variation at this locus is present in all but a very few colonies. When subdivisions of five large populations were studied, genetic differentiation was found within two of these populations. Gene frequencies tended toward the limits of zero or one in populations from relatively undisturbed sites but tended toward intermediate values in the more disturbed ruderal populations presumed to be recently colonized. It is concluded that recently colonized populations are not always genetically uniform.

Estimation of heritability and response to selection for cut-flower yield in gerbera

SummaryHeritabilities and responses to selection for cut flower yield were estimated for a population of gerbera. Broad-sense heritabilities averaged 42 percent, but may be as high as 66 percent. Narrow sense heritabilities averaged 68 percent when based on half-sib families, and averaged 60 percent when based on parent-offspring regression. These results suggest that most genetic variance for flower yield is additive.However, estimates of realized heritability averaged only 16 percent and inbreeding depression was estimated to be at least 38 percent, suggesting a major role for non-additive genetic variance. Nonetheless, selection for cut flower yield is expected to be successful.

Heritability of cut-flower vase longevity in Gerbera

SummaryEstimates of broad-sense heritability for cut-flower vase longevity were 36 and 46 percent for a sample of Gerbera clones. Estimates of narrow-sense heritability for vase longevity were 0, 24 and 38 percent over 3 generations of the Davis Population. Response to selection for this character in this population is expected to be slow.

Quantitative analysis of correlations among flower traits in Gerbera hybrida Compositae : 1. Genetic and environmental correlations.

SummaryPhenotypic (rp), genetic (rg), and environmental, (re) correlations were estimated for 38 flower traits in the Davis population of gerbera (Gerbera hybrida, Compositae). Fifty-two percent of rp and 38% of rg were statistically significant at P < 0.05. Significant negative rp were infrequent, but significant negative rg occurred in 10% of cases. There was a negative correlation between estimates of rg and re, resulting in cases where rg was significant, but rp was not. Individual traits varied in their tendency to correlate phenotypically or genetically, and positively or negatively, with other traits. Traits within the same morphological category, such as disk florets or transitional florets, were more highly intercorrelated than were traits from different categories.

Quantitative analysis of correlations among flower traits in Gerbera hybrida, Compositae. 2. Direct and mean correlated response to selection.

SummaryA Mean Correlation Response (MCR) model was developed to estimate the relative effectiveness of direct selection when other traits also respond to the selection. A measure of the relative effects of mean correlated response and direct response (R) and a measure of the relative efficiency of direct selection (IE) were applied to a genetic correlation matrix of 38 traits. These were measurements of inflorescence, receptacle and involucre, scape, disk florets, ray florets, and trans florets in the Davis population of Gerbera hybrida, Compositae. Generally, traits with high heritability had high direct and mean correlated response; these were often traits measuring disk and trans florets. Traits with low heritability had low direct and mean correlated response; these were often traits measuring the inflorescence. Traits of the inflorescence had the lowest efficiency of direct to mean correlated response.

The estimation and use of selective values in predicting population change.

Received 26.ix.65

Heritability of inflorescence and floret traits in gerbera

SummaryHeritability, repeatability, and components of variance were estimated for 68 morphological characters describing the inflorescence, scape, and florets of Gerbera hybrida. The heritability estimates were generally moderate to high for simple dimension characters and characters which were the mean of several measurements, low to moderate for ratio and composite characters, and low for enumeration characters and characters describing the intraplant variation of traits. The repeatability estimates were generally low for intraplant variation characters, moderate for simple ratio characters, and largest for composite and mean characters. For most of the characters, the variance among inflorescences from the same plant was the largest portion of the total phenotypic variance.

Components of genetic variation for cut-flower yield in the Davis Population of Gerbera

SummaryGenetic variances for cut-flower yield in Gerbera hybrida were partitioned into additive and dominance components. Mean additive variance was 37.3 with a standard error of 12.9. Mean dominance variance was 12.7 with a standard error of 32.6. The dominance ratio was 0.34. Narrow sense heritability, estimated from these components of variance, averaged 0.42 with a standard error of 0.13. Heritabilities estimated from parent-offspring regression averaged 0.25. Gain from selection was less then expected; realized heritabilities were approximately 0.20.