Validation and Characterization of a Seed Number Per Silique Quantitative Trait Locus qSN.A7 in Rapeseed (Brassica napus L.).

Jiaqin Shi,Xiaoxiao Zheng,Jiangjiang Zhang,Hanzhong Wang,Jiepeng Zhan,Jiang Ye,Xinfa Wang,Guihua Liu,Yaoyao Zhu

doi:10.3389/fpls.2020.00068

Abstract

Seed number is a key character/trait tightly related to the plant fitness/evolution and crop domestication/improvement. The seed number per silique (SNPS) shows a huge variation from several to more than 30, however the underlying regulatory mechanisms are poorly known, which has hindered its improvement. To answer this question, several representative lines with extreme SNPS were previously subjected to systematic genetic and cytological analyses. The results showed that the natural variation of seed number per silique is mainly controlled by maternal and embryonic genotype, which are co-determined by ovule number per ovary, fertile ovule ratio, ovule fertilization rate, and fertilized ovule development rate. More importantly, we also mapped two repeatable quantitative trait loci (QTLs) for SNPS using the F2:3 population derived from Zhongshuang11 and No. 73290, of which the major QTL qSN.A6 has been fine-mapped. In the current study, the near-isogenic lines (NILs) of qSN.A7 were successfully developed by the successive backcross of F1 with Zhongshuang11. First, the effect of qSN.A7 was validated by evaluating the SNPS of two types of homozygous NILs from BC3F2 population, which showed a significant difference of 2.23 on average. Then, qSN.A7 was successfully fine-mapped from the original 4.237 to 1.389 Mb, using a BC4F2 segregating population of 2,551 individuals. To further clarify the regulatory mechanism of qSN.A7, the two types of homologous NILs were subjected to genetic and cytological analyses. The results showed that the difference in SNPS between the two homologous NILs was determined by the embryonic genotypic effect. Highly accordant with this, no significant difference was observed in ovule number per ovary, ovule fertility, fertilization rate, and pollen fertility between the two homologous NILs. Therefore, the regulatory mechanism of qSN.A7 is completely different from the cloned qSS.C9 and qSN.A6. These results will advance the understanding of SNPS and facilitate gene cloning and molecular breeding in Brassica napus.

Highlights

Oilseed rape (Brassica napus L.) is one of the most important crops in the world in which production ranks eighth among all crops in 2018
Radoev et al (2008) used the NCII model with six rapeseed accessions to investigate the inheritance of seed number per silique in rapeseed, the results showed that this trait was governed by non-additive effects
The results showed that the seed number per silique of hybrid F1 was mainly determined by the general combining ability (63%) of the parents, followed by the special combining ability (37%) of parental combination, which was mainly determined by the additive effect, followed by the dominant effect (Yang et al, 2017)

Summary

Introduction

Oilseed rape (Brassica napus L.) is one of the most important crops in the world in which production ranks eighth among all crops in 2018 (https://apps.fas.usda.gov/psdonline). Yield is an essential trait for the genetic improvement of most crops (Shi et al, 2009). In the past decades of years, the yield of rapeseed increased very slowly (annual average growth rate = 0.93%), in comparison with its fast improvement in other crops, such as soybean [average annual growth rate (AAGR) = 2.91%] and maize (AAGR = 1.49%) (https://apps.fas.usda.gov/psdonline). The previous study showed that the seed number per silique is tightly correlated with seed yield (Raboanatahiry et al, 2018; Khan et al, 2018). The seed number per silique showed a relatively high heritability, which makes it an important target for selection (Shi et al, 2009; Basunanda et al, 2010; Shi et al, 2015)

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