Tian Shujuan, Ge Jie, Ai Gongli, Jiang Jiao, Liu Qiyan, Chen Xiner, Liu Man, Yang Jianqiang, Zhang Xian, Yuan Li
State Key Laboratory of Crop Stress Biology for Arid Areas, College of Horticulture, Northwest A&F University, Yangling, 712100, Shaanxi, China.
Hortic Res. 2021 Dec 1;8(1):256. doi: 10.1038/s41438-021-00687-9.
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 F 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 F 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. F population genetic analysis indicated that the "148" watermelon line is a homozygous translocation and that the less seed phenotype of the F 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.
无籽是西瓜（Citrullus lanatus）育种中一项重要的农艺性状。传统的少籽西瓜主要是三倍体，由于基因组含量不平衡存在诸多缺点。当某些生殖基因发生突变或通过染色体易位可在二倍体水平实现少籽西瓜，这会导致减数分裂过程出现缺陷。然而，二倍体少籽西瓜的形成机制在很大程度上仍不清楚。在此，我们鉴定出一个自发突变系，西瓜品系“148”，其自花授粉时能正常结籽。总共148×JM F1杂交植株的种子数分别降至两个亲本系种子数的50.3%和47.3%，被认为是少籽的。花粉活力检测和杂交实验表明，F1杂种产生半不育花粉和胚珠。进一步的细胞学观察表明，半不育是染色体相互易位的结果，在减数分裂前期I出现由四条染色体组成的一个四价环。RT-qPCR分析间接证实，半不育表型是由染色体易位而非特定减数分裂基因表达的破坏引起的。F2群体遗传分析表明，“148”西瓜品系是纯合易位，F1杂种的少籽表型是由一个染色体片段易位导致的。通过全基因组重测序和遗传图谱克隆程序，将易位片段进一步精细定位到6号染色体上一个2.09 Mb的区域。我们的研究表明，6号染色体上一个2.09 Mb的染色体片段易位，在减数分裂中期I导致减数分裂缺陷，从而产生二倍体少籽西瓜。我们的研究结果为二倍体水平的少籽西瓜育种提供了一种新的有前景的方法，也为通过人工诱导染色体易位培育少籽西瓜提供了片段大小参考。
