Nathanael R. Hauck

Primary structural features of the S haplotype-specific F-box protein, SFB, in Prunus

The gene SFB encodes an F-box protein that has appropriate S-haplotype-specific variation to be the pollen determinant in the S-RNase-based gametophytic self-incompatibility (GSI) reaction in Prunus (Rosaceae). To further characterize Prunus SFB, we cloned and sequenced four additional alleles from sweet cherry (P. avium), SFB1, SFB2, SFB4, and SFB5. These four alleles showed haplotype-specific sequence diversity similar to the other nine SFB alleles that have been cloned. In an amino acid alignment of Prunus SFBs, including the four newly cloned alleles, 121 out of the 384 sites were conserved and an additional 65 sites had only conservative replacements. Amino acid identity among the SFBs ranged from 66.0% to 82.5%. Based on normed variability indices (NVI), 34 of the non-conserved sites were considered to be highly variable. Most of the variable sites were located at the C-terminal region. A window-averaged plot of NVI indicated that there were two variable and two hypervariable regions. These variable and hypervariable regions appeared to be hydrophilic or at least not strongly hydrophobic, which suggests that these regions may be exposed on the surface and function in the allele specificity of the GSI reaction. Evidence of positive selection was detected using maximum likelihood methods with sites under positive selection concentrated in the variable and hypervariable regions.

Accumulation of Nonfunctional S-Haplotypes Results in the Breakdown of Gametophytic Self-Incompatibility in Tetraploid Prunus

The transition from self-incompatibility (SI) to self-compatibility (SC) is regarded as one of the most prevalent transitions in Angiosperm evolution, having profound impacts on the genetic structure of populations. Yet, the identity and function of mutations that result in the breakdown of SI in nature are not well understood. This work provides the first detailed genetic description of the breakdown of S-RNase-mediated gametophytic self-incompatibility (GSI) in a polyploid species that exhibits genotype-dependent loss of SI. Genetic analyses of six natural sour cherry (Rosaceae, Prunus cerasus) selections identified seven independent, nonfunctional S-haplotypes with disrupted pistil component (stylar-S) and/or pollen component (pollen-S) function. A genetic model demonstrating that the breakdown of SI in sour cherry is due to the accumulation of a minimum of two nonfunctional S-haplotypes within a single individual is developed and validated. Our finding that sour cherry is SI when only one nonfunctional S-haplotype is present has significant evolutionary implications since nonfunctional S-haplotypes would be maintained in the population without causing an abrupt shift to SC. Furthermore, we demonstrate that heteroallelic sour cherry pollen is self-incompatible, which is counter to the well-documented phenomenon in the Solanaceae where SC accompanying polyploidization is frequently due to the SC of heteroallelic pollen.

Molecular and Genetic Analyses of Four Nonfunctional S Haplotype Variants Derived from a Common Ancestral S Haplotype Identified in Sour Cherry ( Prunus cerasus L.)

Tetraploid sour cherry (Prunus cerasus) has an S-RNase-based gametophytic self-incompatibility (GSI) system; however, individuals can be either self-incompatible (SI) or self-compatible (SC). Unlike the situation in the Solanaceae, where self-compatibility accompanying polyploidization is often due to the compatibility of heteroallelic pollen, the genotype-dependent loss of SI in sour cherry is due to the compatibility of pollen containing two nonfunctional S haplotypes. Sour cherry individuals with the S4S6S36aS36b genotype are predicted to be SC, as only pollen containing both nonfunctional S36a and S36b haplotypes would be SC. However, we previously found that individuals of this genotype were SI. Here we describe four nonfunctional S36 variants. Our molecular analyses identified a mutation that would confer loss of stylar S function for one of the variants, and two alterations that might cause loss of pollen S function for all four variants. Genetic crosses showed that individuals possessing two nonfunctional S36 haplotypes and two functional S haplotypes have reduced self-fertilization due to a very low frequency of transmission of the one pollen type that would be SC. Our finding that the underlying mechanism limiting successful transmission of genetically compatible gametes does not involve GSI is consistent with our previous genetic model for Prunus in which heteroallelic pollen is incompatible. This provides a unique case in which breakdown of SI does not occur despite the potential to generate SC pollen genotypes.