Testing antimitotic agents to induce diploid gametophytes for the production of sterile triploid sporophytes in sugar kelp (Saccharina latissima)

Abstract The development of non-reproductive seaweeds could alleviate environmental concerns associated with the upscaling of seaweed aquaculture and the deployment of genetically improved cultivars containing non-local genetic material. However, current means to produce triploids rely on diploid gametophytes retrieved via apospory, a rare and poorly controlled process that may yield gametophytes with unclear sex differentiation. Antimitotic agents like colchicine and oryzalin could potentially offer an alternative route to induce diploid gametophytes that can serve as a foundation for sporophyte triploidy. Here, we assessed the effectiveness of colchicine and oryzalin in inducing diploidy in Saccharina latissima gametophytes. Single-cell gametophytes were treated at multiple developmental stages shortly after spore germination and for a range of exposure durations. Cell and nuclear development during the first cell cycle, together with treatment effects on gametophyte growth, were analyzed to identify the narrow mitotic window during which chromosome doubling was expected to occur, and to guide the selection of optimal treatment conditions for diploidy verification. The first mitotic division occurred within three days after spore germination in males and within five days in females, revealing sex-specific differences in optimal treatment timing. Although both antimitotic agents affected gametophyte viability and growth, neither colchicine nor oryzalin verifiably induced stable diploidy under current treatment conditions. Together with technical challenges associated with large-scale gametophyte diploidy verifications, these findings indicate that the large-scale application of antimitotic agents to induce kelp gametophyte diploidy remains currently impractical.