A 2.09 Mb fragment translocation on chromosome 6 causes abnormalities during meiosis and leads to less seed watermelon

Abstract

Seedlessness is a valuable agronomic trait in watermelon (Citrullus lanatus) breeding. Conventional less seed watermelons are mainly triploid, which has many disadvantages due to unbalanced genome content. Less seed watermelon can be achieved at the diploid level when certain reproductive genes are mutated or by chromosome translocation, which leads to defects during meiosis. However, the formation mechanism of diploid less seed watermelons remains largely unknown. Here, we identified a spontaneous mutant line, watermelon line “148”, which can set seeds normally when self-pollinated. A total of 148 × JM F1 hybrid plants exhibited seed number reductions to 50.3% and 47.3% of those of the two parental lines, respectively, which are considered to be less seed. Examination of pollen viability and hybridization experiments revealed that F1 hybrids produce semisterile pollen and ovules. Further cytological observations indicated that semisterility was a result of a reciprocal translocation of chromosomes, which exhibited one quadrivalent ring of four chromosomes at prometaphase I during meiosis. RT-qPCR analysis indirectly confirmed that the semisterile phenotype is caused by chromosome translocation rather than disruption of specific meiotic gene expression. F2 population genetic analysis indicated that the “148” watermelon line is a homozygous translocation and that the less seed phenotype of the F1 hybrid is prompted by one chromosome fragment translocation. The translocated fragment was further fine mapped to a 2.09 Mb region on chromosome 6 by whole-genome resequencing and genetic map cloning procedures. Our work revealed that a 2.09 Mb chromosome fragment translocation on chromosome 6, causing meiotic defects at metaphase I during meiosis, leads to diploid less seed watermelon. Our findings provide a new promising method for less seed watermelon breeding at the diploid level, as well as a fragment size reference for breeding less seed watermelon through artificially induced chromosome translocation.

Low Female Gametophyte Fertility Contributes to the Low Seed Formation of the Diploid Loquat [Eriobotrya Japonica (Thunb.) Lindl.] Line H30-6

“…That was to say, the abnormal meiosis of H30-6 was not caused by polyploidization. A recent report showed that a 2.09 Mb chromosome fragment translocation causes abnormalities during meiosis and leads to pollen semisterility and fewer seeds in watermelon (Tian et al, 2021 ). Additionally, the fragment translocation results in one quadrivalent ring of four chromosomes at prometaphase I during meiosis.…”

Genic male and female sterility in vegetable crops

“…Potato ( Solanum chacoense ) ScRALF3 participates in both male and female gametophyte development. Scralf3 mutants produced pollen grains with reduced viability and fewer seeds, primarily due to abnormal nuclear distribution and asynchronous nuclear divisions in the embryo sac [ 120 , 121 ].…”

Promoter variations of ClERF1 gene determines flesh firmness in watermelon

“…The GWAS results of fruit firmness in Pyrus revealed that an identical-by-descent segment harboring a 12 bp insertion in TIC55 determined fruit softening [ 16 ]. After the release of watermelon genome [ 17 , 18 ], functional genomic studies for important quality and storability relevant traits are also facilitated [ 19 – 22 ]. ClERF4 gene influencing fruit rind hardness and cracking was fine-mapped through BSA-seq in watermelon [ 20 ].…”