Mitotic defects lead to unreduced gamete formation

Polyploidy is a major driver of speciation and evolutionary changes in plants and animals. Production of unreduced gametes is considered as a main pathway for polyploid formation. However, the precise molecular mechanisms underlying unreduced gamete production, particularly those arising from mitotic defects of spermatogonia/oogonia, remain poorly understood. Here, a heterozygous espl1 (extra spindle pole bodies like 1) knockout line was generated in diploid loach (Misgurnus anguillicaudatus) by CRISPR/Cas9 technology. Interestingly, we found about 90% diploids and 10% triploids in the progeny of espl1+/- males and wild-type (WT) diploid females. espl1+/- male could produce 1n sperms, along with a certain volume of unreduced sperms (2n). We showed that the heterozygous loss of espl1 was enough to induce spermatogonial mitotic sister chromatid separation defects, causing the production of unreduced sperms. Notably, our results provide new strategies for the aquaculture-oriented polyploid breeding.