Digging out molecular markers associated with low salinity tolerance of Nannochloropsis oceanica through bulked mutant analysis

Abstract

Nannochloropsis oceanica is a marine microalgal species with both economic value and biological importance. It grows fast, contains rich oils, reproduces asexually, holds a small and haploidy genome, and is easy to be modified genetically. However, the genetic study of N. oceanica is scarce. Very less genetic bases of its traits have been deciphered, and no gene has been isolated from it with the function verified simultaneously via either genetic or reverse genetic approaches or both (de novo cloned). Changing medium salinity may aid to control harmful organisms met during large scale cultivation. As a stress, it may also facilitate the accumulation of desirable chemicals including fatty acids. In order to decipher the genetic basis of the low salinity tolerance of N. oceanica, we mutated N. oceanica with Zeocin. In total, five mutant bulks were constructed at equal number of cells, 100 mutants each, which were tolerant to a discontinuous serial of salinities from that of 100% of f/2 to that of a mixture of 4% of f/2 and 94% of BG11. The bulks were genotyped through whole genome re-sequencing and analyzed with bulked mutant analysis (BMA) newly modified from bulked segregant analysis (BSA). In total, 47 SNPs and 112 InDels were found to associate with the low salinity tolerance, and around them a set of low salinity tolerance associating genes were identified. A set of annotatable genes commonly found between control and different salinities indicated that the genes functioning in gene expression, energy metabolism and cellular structure may be involved in the low salinity tolerance. These associating molecular markers and genes around them were not enough for outlining the physiological mechanism underlining the tolerance; however they should aid to improve N. oceanica genetically.