Criterios diagnósticos

Half-sib progeny trialling (HSPT) is a breeding strategy that has been extensively used in breeding perennial forages to test the performance of parental genotypes through replicated progeny trials (Vogel and Pedersen, 1993). When utilizing maternal half-sib families for HSPT, progeny are either topcrossed or polycrossed to generate half-sib families. When either approach is used, breeders often assume that each female parent (recipient parent) has the

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same male tester (donor pollen source). In the case of a polycross, the male tester is made up from a heterogeneous mixture (equal proportions) of pollen from all genotypes within the polycross. Any differences among half-sib family progeny can then be primarily attributed to the genetic potential of the maternal parent and not the genetic contribution of the pollen source (Fehr, 1987).

Results obtained from this experiment show that the maternal parents in each isolation cage did not have the same male tester pollen (Figure 3.10). While this is not entirely surprising considering that there were no clonal replications per parent genotype within each isolation cage, it was surprising that the bulked maternal half-sib seed from both cages only slightly resolved the differences in paternal contributions among fathers (i.e. still very unbalanced) (Figure 3.9), and that it was far from random mating as illustrated. Perhaps if both clones of each maternal parent had been polycrossed in the same isolation cage (as opposed to the two separate cages), a better balance of paternal contributions may have be achieved as more neighbour to neighbour interactions would have existed. Nevertheless, this result highlights the importance of additional clonal replication when generating half-sib families in white clover.

Whilst the number of replicates required to achieve random mating cannot be simulated with the available data set, one could speculate that many replicates would be required judging by Figure 3.9. For a small number of entries, lattice or alpha designs are most likely required to ensure random mating as proposed by Morgan (1988) for wind pollinated species. For larger numbers of parents (≤50), Wright (1965) developed suitable polycross designs. Regardless of whether the plant species is insect or wind pollinated, polycross replication seems

fundamental for the successful generation of ‘true’ half-sib families in forage species. Topcrossing with male rows using the base population or the previous cycle as the pollen tester is likely to be a more accurate approach for generating true half-sib families for HSPT (Posselt, 2010). At least with this approach, the tester will provide an excess of pollen, which will predominate the pollen source (Posselt, 2010) and eliminate any differences in testers between the evaluated half-sib progeny.

The low level correlation between maternal seed yield and number of fathers per half-sib family (Appendix A.4) indicates improving seed yield per clone would marginally help to reduce unbalanced male testers within non-replicated polycrosses. This tends to improve further if a component of total seed yield (number of harvested inflorescences) is increased per plant. The above is not surprising, considering genotypes with more inflorescences would

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promote a greater number of pollinator visits as discussed earlier and hence interactions among more donor parents. Extremely low yielding genotypes may have been pollinated by a limited number of pollinator visits, explaining the poor number of donor parents in the

progeny (Figure 3.10; genotype 14, cage 1). However, despite a slight improvement in the number of donor parents per half-sib family in higher seed yielding genotypes, even

genotypes with high seed yields (i.e. Figure 3.10; genotypes 4 and 5 in isolation cage 1) still had relatively few pollen donors compared to the total available. It must be acknowledged that the number of paternal donors did not increase substantially as seed yield increased, but the contributions from the limited number of paternal donors were more uniform (Figure 3.10; genotype 4 isolation cage 1).

For practical purposes, the best polycross system would be likely to include an increase in replications, seed yield and number of inflorescences per genotype to improve random mating. This could possibly be achieved in field plots, where bigger plants can be managed more successfully. An increase in plant size and number would in turn support the

requirements of a complete nucleus hive of bees. Increasing pollinator density is more manageable with complete hives and has been shown to positively influence seed yield in white clover using honey bees (Forster, 1974). Larger crosses may also facilitate the use of honey bees (Apis mellifera), which have been shown to not only increase seed yield per cage (Cecen et al., 2007) but also visit fewer florets per inflorescence and travel greater distances between succeeding inflorescences (MichaelsonYeates et al., 1997). If a pot system was still desired for convenience, randomizing the plants throughout the duration of crossing could help alleviate the nearest neighbour effect. Again, the effect of pollinator density and or species would also be worth investigating, as well as the duration of crossing.

In terms of other downstream applications, pollination is likely to have a pronounced

influence on synthetic performance as well. First generation synthetics are generally an even blend of maternal seed yield from the inter-pollinated parents (Fehr, 1987). Equal

contributions of maternal seed are bulked to limit undesirable levels of inbreeding during generation advance, often a concern of breeders in narrow based synthetics (Rumbaugh et al., 1988). However, unbeknown to the breeder, non-random mating may result in the same problem as uneven bulking of maternal half-sib seed, despite the best practices used. Perhaps equal bulking of a diallel cross between parents in the Syn0 as described by Fehr (1987) and carried out by Piano et al. (2007), is a viable method to mitigate such a problem in the first generation synthetic of non-random mating sensitive species like white clover when theoretically all of the inbreeding takes place (Busbice, 1969).

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