The Dominant Ms Allele in Onion Shows Reduced Penetrance

Abstract

<jats:p>The most commonly used source of cytoplasmic male sterility in onion (<jats:italic>Allium cepa</jats:italic>) is controlled by the interaction of the cytoplasm [male-sterile (S) or normal male-fertile (N)] and one nuclear male-fertility-restoration locus (<jats:italic>Ms</jats:italic>). Scoring of genotypes at <jats:italic>Ms</jats:italic> is generally done by testcrossing male-fertile to male-sterile (S <jats:italic>msms</jats:italic>) plants, followed by scoring of testcross progenies for male-fertility restoration. We identified two N-cytoplasmic families, one that was homozygous dominant and the other segregating at <jats:italic>Ms</jats:italic>. Plants from each of these two families were individually testcrossed to male-sterile onion. Nuclear restoration of male fertility in testcross progenies was evaluated in the field over 4 years. For male plants homozygous dominant at <jats:italic>Ms</jats:italic>, we expected testcross families to show 100% male-fertility restoration, but observed mean values between 46% and 100%. For plants segregating at <jats:italic>Ms</jats:italic>, we again observed lower than expected frequencies of male-fertility restoration. These results demonstrate that the dominant <jats:italic>Ms</jats:italic> allele shows reduced penetrance, requiring that male-fertility restoration be scored over years to more confidently assign genotypes at <jats:italic>Ms</jats:italic>.</jats:p>

Variable Penetrance among Different Sources of the Male Fertility Restoration Allele of Onion

“…For AC, frequencies of male fertile plants in testcross families carrying the dominant Ms allele agreed with predictions of the AcPms1 marker. However significantly lower pollen stainability was observed relative to Ski (Table 1), consistent with reduced penetrance of the dominant Ms allele from AC, as previously reported by Melgar and Havey (2010). Male sterility in inbred B2354A has been problematic across many seed production cycles.…”

“…AC43 was crossed as the male with BYG15-23 (N-cytoplasmic and homozygous recessive at Ms) and a segregating F 2 family was developed (King et al, 1998). F 2 progenies were self-pollinated and testcrossed to Scytoplasmic male sterile plants, and one F 3 family (USDA breeding plot 16278) homozygous dominant at Ms was selected and used by Melgar and Havey (2010) for three generations, producing an inbred (AC) that possesses S cytoplasm and is heterozygous at Ms (Fig. 1).…”

“…Historically, it has been timeconsuming to select against dominant alleles at Ms from onion populations because male fertile plants must be testcrossed as males to male sterile plants and testcross progenies evaluated for male fertility. This process may not be conclusive because the dominant allele at Ms may show reduced penetrance (Melgar and Havey, 2010). The elimination of dominant Ms alleles has become easier with the use of robust molecular markers in linkage disequilibrium with Ms (Havey and von Kohn, 2017;Kim et al, 2015), allowing breeders to genotype and select plants before flowering and crossing.…”

Partial success of marker-assisted selection of ‘A’ and ‘B’ onion lines in Brazilian germplasm

“…In another study with Indian germplasm, Khar and Saini (2016) reported a low agreement between the markerassisted selection of 'A' and 'B' lines with viability test with 0.5% acetic carmine. The low agreement reported by the authors mentioned above may be related to the reduced penetrance of the Ms allele reported by Melgar and Havey (2010), in which plants homozygous dominant at Ms may not present 100% fertility restoration, suggesting not a failure of the marker-assisted selection. In the present study, agreement was observed for the classification by the pollen viability methods employed with assisted selection for the plants of seven accessions and absence of agreement for plants of five accessions, indicating reasonable accuracy of the assisted selection.…”

Plant Breeding: Past, Present and Future

“…However, as recently as 2010, Melgar and Havey (2010) have confirmed that the dominant Ms allele can show reduced penetrance, requiring that male-fertility restoration be scored over years to confidently assign genotypes at Ms. Male-sterile plants have composition S-msms, the thermostabile sterility (no pollen under high temperatures) resulting from premature breakdown of the tapetum.…”