3.3 Methods
3.5.5 Trait associations and cluster analyses
Genetic shifts and trait associations identified for some traits were cultivar specific, implying these cultivars did not shift towards similar phenotype expression after surviving at Poukawa (Figures 3.7-3.10).
Trait associations did not appear to change for Commando populations with similar vector angles between plant habit score, herbage growth score, and leaf width score in both PCA analyses (Figures 3.7 and 3.8). Further, the number of cluster groups identified did not change between these populations (Figures 3.7 and 3.8, Tables 3.3 and 3.4). This may indicate that the Commando population surviving in Poukawa did not differ substantially from the overall mean of the Original population sown. Therefore survival of Commando plants at Poukawa was not enhanced by advantageous phenotypes. These results may relate to the development of this cultivar for the seed industry. Stability of cultivar performance during large scale seed production is important for obtaining plant variety rights (Plant Variety Rights Act, 1987). Genotypes in this cultivar would have been polycrossed in the field during seed production, and the resulting progeny would be required to show uniform phenotypes by industry before becoming commercially
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available. Therefore, our results showing mostly uniform trait associations could be a result of prioritising uniform phenotypes during cultivar development. For further persistence research, the progeny from the Commando cultivar may not offer insight into genetic factors contributing to improved persistence.
For both Samson populations, herbage growth score and plant habit score showed strong positive associations between vectors (Figures 3.9 and 3.10). The Original population showed leaf width scores were negatively associated with herbage growth and plant habit (Figure 3.9), whereas the Persistent population showed aftermath heading was negatively associated with these traits (Figure 3.10). A genetic shift was identified in Samson due to differences in aftermath heading scores and leaf width scores between populations. Further, the Original population had less cluster groups identified than the Persistent population (Figures 3.9 and 3.10, Tables 3.5 and 3.6). These results imply the half-sibling progeny of Samson Persistent could be used to further study genetic factors associated with persistence.
Two options are suggested in using the five cluster groups identified in Persistent half-sibling families for future work. The first, is selection of half-sibling families associated with traits linked to persistence. Herbage growth score, plant habit score, and aftermath heading were found to have medium to high estimates of narrow sense heritability conveying the opportunity for genetic gains in these traits with further breeding (Table 3.1 and 3.2). Selecting half-sibling families with more prostrate plant habits (grazing avoidance), high herbage growth, and low aftermath heading would further germplasm advancement. Although greater aftermath heading scores were
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identified as possibly contributing to grazing tolerance and improved persistence, it is not advised this trait be promoted for cultivar development. Low levels of aftermath heading are preferable for forage cultivars as it greatly improves seasonal yield and quality (Soper & Mitchell, 1956; Wilkins, 1991; Lee, et al., 2012). Cluster analysis identified groups 2 and 4 had half-sibling families with above average herbage growth score, the most prostrate plant habits, and below average aftermath heading for half-sibling families within the Samson Persistent population (Figure 3.10, Table 3.6.). Polycrossing these 17 half-sibling families, and assessing progeny using multi-site field trials would be beneficial in developing a breeding pool for introgressing enhanced persistence into elite lines.
The second option for further research would be to use plants from both Samson Persistent and Original half-sibling families to further characterise the genetic effect on persistent phenotypes under different environments. The process would involve extracting tillers from each half-sibling family and growing clonal plants. Establishing clonal plants from both populations of each genotype into two space plant field trials in different environments: low stress environment with optimum conditions, and a high stress dry environment. Collection of two to three years of seasonal morphological data on traits of interest such as plant habit score, aftermath heading, herbage growth score, tiller density/appearance rate, sheath lengths, heading date, and aftermath heading. Analyse and compare the populations of the clonal phenotypes between the sites, seasons and years. This trial would further characterise genetic and environmental interaction of these traits and identify how persistent phenotypes change under different environments. Results would inform plant breeders if field assessments of breeding lines need to be
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under stressed conditions to accurately identify underlying genetics of desirable phenotypes attributed to improved persistence.
3.6 Conclusions
This study estimated additive genetic variation, family mean narrow-sense heritability, and co-efficients of additive genetic variation of half-sibling families generated from Persistent plants and half-sibling families generated from seed of the Original cultivars ‘Grasslands Samson’ and ‘Commando’. These results showed that genetic shifts over time were cultivar specific suggesting that the cultivar Commando was less subject to directional selection. Genetic shifts identified within the Samson cultivar suggests persistent populations shifted towards traits that have previously been associated with animal grazing avoidance improving survival of plants in the field. Groups of half-sibling families with more prostrate plant habits, low aftermath heading, and high herbage growth were identified through trait association and cluster analysis as potential germplasm to enhance persistence.
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