BIBLIOGRAFÍA

Whilst there was significant phenotypic variation among families for 11 of the 18 traits measured, variation tended to only be significant among the upper and lower extreme values, and therefore there was a relatively small component of among family

phenotypic variation (VF) for most traits. The family mean repeatability enabled the

estimation of an upper limit of genetic determination (Falconer and Mackay, 1996), and was 51% or less for all of the traits. This result indicated that among family variance made up a relatively low proportion of the total phenotypic variance, and therefore a low estimated heritability among family groups in this hybrid population is expected. Since the attributes were measured in a single environment, there is no measurement as to the magnitude of family-by-environment variation, and therefore care should be taken as to the interpretation of repeatability estimates as broad sense heritability.

Interspecific hybrid populations are expected to offer increased variability as a result

of extreme heterozygosity in F1 hybrids. Each genotype in the F2 and later generations

is likely to differ from each other individual for many characteristics, with some segregants showing phenotypes that could not be predicted from the morphology or

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physiology of the parental species (Allard, 1999). Each genotype in a family represents an individual hybridisation event and as such may have alien chromosomes and/or chromosomal segments introgressed to varying degrees on differing chromosomes, and thus, the widely segregating nature of the population can largely explain the relative lack of among family variation as compared to the variation among genotypes. Lack of variation among families could also be attributed to sampling bias, where only six plants (three per rep) from each hybrid family were included in the design, meaning that the results presented here may not be a true reflection of the represented

families. However, the early generation hybrid nature of the material in the present study is likely to be the key determinant for the result showing low phenotypic

variation among BC1F2 families, as compared to among genotype variation.

Stolon diameter was one of the few traits that showed significant variation, both among hybrid families, and between hybrid families and their parental species. There was evidence to suggest possible transgressive segregation within the hybrid

population for this trait, with some families expressing stolon diameters significantly above those recorded for either parental species. Transgressive segregation can be defined as the appearance of individuals in segregating populations that fall beyond their parental phenotypes (Rieseberg et al., 1999, Tanksley, 1993). Transgressive segregation appears to be ubiquitous in plant hybrids, playing a significant role in crop improvement and evolution as it can affect characters of adaptive significance, or characters that allow them to occupy new niches, or better compete in existing

environments (Tanksley, 1993, Lewontin and Birch, 1966, Stebbins, 1950, Rieseberg et al., 1999).

Evidence reported here suggests that transgressive segregation may be occurring in

the BC1F2 population for stolon diameter. Stolon diameter is an important trait related

to persistence of white clover. It has been reported that vegetative persistence in white clover in sub-optimal conditions is generally associated with greater stolon density, which is a complex combination of stolon branching frequency, internode length, leaf size, and stolon thickness (Williams, 1987). Studies by Turner (1990a) (1990b) showed that under drought stress white clover sheds its leaves, while the stolon adjusts its osmotic potential and pressure potential to survive. When water was

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applied again, the white clover recovered from the surviving stolons. Brock and Kim (1994) were able to show that during a drying down phase in a drought experiment (before growth ceased), the leaf production of the smaller, thinner stoloned cultivar Tahora was more affected than that of the thicker stoloned cv. Dusi. It was proposed that the thicker stoloned genotypes were more resistant to heat stress than the cv. Tahora. However, following drought, Tahora was able to recover more quickly than Dusi. It was clear that white clover, regardless of stolon morphology is severely impacted by drought, indicating the complex nature of drought tolerance and the likelihood that many individual traits contribute.

Persistence of white clover can be measured as the survival of the individual plant, or as maintenance of clover content per unit area (Woodfield and Caradus, 1996). Individual stolons generally have short life spans, with less than 10% of stolons surviving longer than a year under New Zealand hill country conditions (Chapman, 1983). Thus continual renewal of the stolon population (by dense stolon branching) has been seen as a way increasing persistence of white clover content in pastures, with plant breeders generally having to balance characteristics for high productivity (large leaf, thick stolons) with those associated with survival under sub optimal conditions (Williams, 1987).

Given the above equivocal evidence, it is unclear whether populations with thick stolons will be better adapted to dry conditions than populations with thinner stolons. Due to the hybrid nature of these populations, it is possible that unique new

physiologies, not measured here, may have been introgressed, and, as such, direct selection under a white clover vegetative persistence model may not be the most efficient plant breeding methodology. Further trialling of this material is required to elucidate the function of increased stolon thickness, particularly in relation to drought stress.

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