Tetrad sterility, in which only clumps of four premature pollen grains are released from anthers, has been observed in some modern potato cultivars. It is a form of cytoplasmic male sterility caused by the cytoplasm derived from the Mexican tetraploid species Solanum stoloniferum Schlechtd. et Bché., an important source of resistance to Potato virus Y in potato breeding. However, since S. stoloniferum is highly polymorphic, the source of tetrad-sterility-causing cytoplasm is unknown among diverse S. stoloniferum accessions. In this study, we directly crossed 24 S. stoloniferum accessions with pollen from 4x S. tuberosum and obtained 39 hybrids from 12 accessions. Nineteen hybrids from six accessions showed tetrad sterility, with either D/γ-or W/γ-type cytoplasm, and were triploid, tetraploid, or hexaploid.The W/γ-type cytoplasm was not necessarily associated with tetrad sterility. Sequence comparisons of 17 mitochondrial genes and their intergenic regions revealed a length polymorphism in the intergenic region between rpl5 and rps10, in which an ampli ed band of 859 bp was associated with tetrad sterility. This speci c cytoplasm causing tetrad sterility is named TSC sto . The 859-bp band would be a useful diagnostic marker for identifying TSC sto in potato breeding.

Background Tetrad sterility in potato is caused by a specific cytoplasm, called TSCsto, derived from the Mexican wild tetraploid species Solanum stoloniferum. Different S. stoloniferum accessions crossed as females with S. tuberosum resulted in 12 fertile hybrids and 27 sterile hybrids exhibiting tetrad sterility. Results Whole-mitochondrial-genome sequencing was performed for two fertile hybrids and three hybrids exhibiting tetrad sterility. Two to seven contigs, with the total assembly lengths ranging from 462,716 to 535,375 bp, were assembled for each hybrid. Unlike for the reference mitochondrial genome (cv. Désirée), two different recombinant-type contigs (RC-I and RC-II) were identified. RC-I featured by the rpl5-ψrps14 gene joined to the nad6 gene, generating a novel intergenic region. Using a PCR marker (P-3), we found that this intergenic region occurred exclusively in interspecific hybrids exhibiting tetrad sterility and in their parental S. stoloniferum accessions. A part of this intergenic sequence was expressed in the pollen. From a large survey in which P-3 was applied to 129 accessions of 27 mostly Mexican wild species, RC-I was found in diploid S. verrucosum and polyploid species. From eight accessions of S. verrucosum used as females, 92 interspecific hybrids were generated, in which only those carrying RC-I exhibited tetrad sterility. Conclusions RC-I was clearly associated with tetrad sterility, and the RC-I-specific intergenic region likely contains a causal factor of tetrad sterility.

Sex expression contributes to fruit quality and yield in the Cucurbitaceae. In melon, orchestrated regulation by sex determination genes explains the mechanism of sex expression, resulting in a great variety of sexual morphologies. In this study, we examined the Japanese weedy melon UT1, which does not follow the reported model of sex expression. We conducted QTL analysis using F2 plants for flower sex on the main stem and the lateral branch and mapped a 'femaleness' locus on Chr. 3 (Fem3.1) and a 'type of flower femaleness' (female or bisexual) locus on Chr. 8 (tff8.1). Fem3.1 included the known sex determination gene <mACS11>. Sequence comparison of CmACS11 between parental lines revealed three nonsynonymous SNPs. A CAPS marker developed from one of the SNPs was closely linked to femaleness in two F2 populations with different genetic backgrounds. The femaleness allele was dominant in F1 lines from crosses between UT1 and diverse cultivars and breeding lines. This study suggests that the identity of tff8.1 is CmCPR5, a recently reported bisexual flower control gene. We found that the Japanese weedy melon UT1 does not follow the conventional sex expression model because of the interaction of the loci Fem3.1 and tff8.1 with the previously reported sex determination genes. The results of this study provide new insights into the molecular mechanisms of sex determination in melons and considerations for the application of femaleness in melon breeding.