Genetics of the polycross : 2. Male fertility variation in norway spruce, Picea abies (L.) Karst

Genomic prediction for hastening and improving efficiency of forward selection in conifer polycross mating designs: an example from white spruce

Genomic selection (GS) has a large potential for improving the prediction accuracy of breeding values and significantly reducing the length of breeding cycles. In this context, the choice of mating designs becomes critical to improve the efficiency of breeding operations and to obtain the largest genetic gains per time unit. Polycross mating designs have been traditionally used in tree and plant breeding to perform backward selection of the female parents. The possibility to use genetic markers for paternity identification and for building genomic prediction models should allow for a broader use of polycross tests in forward selection schemes. We compared the accuracies of genomic predictions of offspring's breeding values from a polycross and a full-sib (partial diallel) mating design with similar genetic background in white spruce (Picea glauca). Trees were phenotyped for growth and wood quality traits, and genotyped for 4092 SNPs representing as many gene loci distributed across the 12 spruce chromosomes. For the polycross progeny test, heritability estimates were smaller, but more precise using the genomic BLUP (GBLUP) model as compared with pedigree-based models accounting for the maternal pedigree or for the reconstructed full pedigree. Cross-validations showed that GBLUP predictions were 22-52% more accurate than predictions based on the maternal pedigree, and 5-7% more accurate than predictions using the reconstructed full pedigree. The accuracies of GBLUP predictions were high and in the same range for most traits between the polycross (0.61-0.70) and full-sib progeny tests (0.61-0.74). However, higher genetic gains per time unit were expected from the polycross mating design given the shorter time needed to conduct crosses. Considering the operational advantages of the polycross design in terms of easier handling of crosses and lower associated costs for test establishment, we believe that this mating scheme offers great opportunities for the development and operational application of forward GS.

Frequency-dependent male reproductive success in a polycross of Douglas fir

Frequency dependence of male reproductive success (RS) was investigated in a polycross of Douglas fir. We expect frequency-dependent RS in conifer polycrosses for two reasons. The first is the presence of "pollen chambers" in conifers, which limits the number of competing pollen grains and enhances the reproduction of poor males when at high frequency in the polymix. The second reason is the possibility of competitive interactions among pollen types, which generally gives more complicated patterns of RS. To measure frequency dependence, we performed four separate polycrosses with respective proportions of 1∶1∶1, 2∶1∶1, 1∶2∶1, and 1∶1∶2 of the same three males. Male RS was measured with isozyme markers in seed offspring, and three different estimation models were applied to the data. A general log-linear model detected frequency-dependent RS, but did not allow any specific characterization of its nature. A comparison of two more specific models, the first incorporating pollen chamber size and the second incorporating competitive interactions, indicated that competitive interactions were predominantly responsible for the observed frequency-dependent RS. The implications of frequency-dependent RS for forest tree breeding are discussed.

Joint estimation of immigration and mating system parameters in gymnosperms using the EM algorithm

An EM algorithm procedure is presented for the maximum-likelihood joint estimation of immigration and mating system parameters of mixed-mating system models for gymnosperms. In addition to accommodating multiallelic and multilocus data in mature populations and pollen pools, the EM estimates are insensitive to allelic frequency changes in foreign population and may approach closer to global maximum-likelihood estimates with each iteration, regardless of initial starting values. Estimates of rates of selfing (Ŝ), outcrossing (Ô), and immigration (Î) derived from the model are bounded strictly within the natural biological range (i.e., 0 ≤ Ô + Î ≤ 1; Ŝ + Ô + Î = 1).

Differential male reproductive success in Douglas fir

Differential male reproductive success was studied in clones at two seed orchards of Douglas fir, Pseudotsuga menziesii (Mirb.) Franco. The performance of tester pollen parents was compared in controlled pollinations with two-parent pollen mixes. Marker pollen homozygous for a rare IDH allele was the genetic marker in each pollen mix. The resulting seeds were analyzed electrophoretically. At both seed orchards, the proportion of seeds sired by tester pollen significantly varied among the tester pollen parents. Tester pollen parents did not perform the same across all seed parents. The significant interaction effect was evidence of male-female complementarity. These results suggest a genetic basis to differential male reproductive success in Douglas fir.