Abstract
Known since ancient times, flat-shaped peach (Prunus persica L. Batsch) fruits have aroused considerable research interest. Breeding activities for this trait have expanded since the 1980s of the last century in several countries, in parallel with research into its genetic basis. Following the need for molecular markers to assist selection, linkage mapping studies have positioned the flat shape S locus at the end of chromosome 6. Recently, a series of independent studies focused on detailed characterization of the genomic region harbouring the S locus and different hypotheses about the candidate gene or variant were proposed: from a constitutively activated cell death 1 protein (PpCAD1) to a Brassinosteroid insensitive 1-associated receptor kinase 1 group (PpLRR-RLK) until the discovery of large structural variant (a chromosomal inversion of about 1.7 Mb) putatively affecting an OVATE Family Protein (PpOFP1) and a homolog of sucrose non-fermenting 1-related kinase (PpSNF4), respectively, located at the proximal and distal breakpoints. This short review revises historical studies and recent literature, integrating reanalysis of available genetic and genomic data, to provide a critical overview of the topic and highlight avenues for further research.
Highlights
Reported in the Middle Age book Luo Yang Hua Mu Ji of the author Zhou Shi Hou (1081 AD), the flat shape of peach fruits (‘Pan Tao’ in Chinese) has long been a botanical interest in Occidental countries (Huang et al 2008)
A series of independent studies focused on detailed characterization of the genomic region harbouring the S locus and different hypotheses about the candidate gene or variant were proposed: from a constitutively activated cell death 1 protein (PpCAD1) to a Brassinosteroid insensitive 1-associated receptor kinase 1 group (PpLRR-RLK) until the discovery of large structural variant putatively affecting an OVATE Family Protein (PpOFP1) and a homolog of sucrose non-fermenting 1-related kinase (PpSNF4), respectively, located at the proximal and distal breakpoints
This approach led to a series of publications identifying at least four different candidate genes for the S trait: Prupe.6G292200, having homology with a constitutively activated cell death 1 protein (Cao et al 2016); Prupe.6G281100, similar to a leucine-rich repeat receptor-like kinase orthologous to the Brassinosteroid insensitive 1-associated receptor kinase 1 group (PpLRR-RLK) (López-Girona et al 2017); Prupe.6G290900 encoding a putative OVATE Family Protein (PpOFP1) (Zhou et al 2021; Guo et al 2020; Guan et al 2021); and Prupe.6G323700, homologous of sucrose non-fermenting 1-related kinase, (PpSnRK1βγ/PpSNF4) (Guo et al 2021)
Summary
The first genetic characterization dates back to Lesley (1940), who described the inheritance of the flat (saucer) trait as Mendelian, monogenic and dominant over round shape (S/s). Following the release of the peach genome sequence (Verde et al 2013), building upon the chromosomal position of the S locus (spanning about 3 Mb) and the strong association of the flat trait with some SSR markers, different research groups moved from classical genetics to genome-wide approaches in search of the causal gene(s) and/or causal variant(s) (Micheletti et al 2015). This reconstruction of recombination events is in agreement with MA040a and S positions in the map shown by Lambert et al (2016) Such interval de facto excludes the candidate variant within PpLRR-RLK (a deletion spanning about 10 Kb upstream of the start codon and 693 bp downstream), proposed by López-Girona et al (2017) based on co-segregation with flat trait in a panel of 246 cultivars. The size of the replaced region is too wide to be useful for restriction of the S locus
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