Sexual compatibility in giant kelp gametophytes: inter-cultivar hybridization is average between parents but excels under harsher conditions

In hybridization experiments, productivity improvement and modification of morphologies and chemical compositions were recently demonstrated for Macrocystis sporophytes in the Southeastern Pacific. However, the plasticity for gametogenesis environmental requirements in hybrid crosses has not been studied, even though such plasticity is a key aspect to design kelp breeding programs or to implement restoration set-ups. Effects of temperature (9–18 °C) and photon irradiance (10–120 μmol photons m−2 s−1) were estimated for reproductive success (RS) and sex ratio in an integrifolia × pyrifera inter-cultivar cross. As starting materials for breeding, we used (i) clonal gametophytes (i.e., strains from culture collection germplasms) and (ii) sporulation products (e.g., gametophytes from sporophylls), producing inter-locality crosses and their respective intra-locality controls. In germplasm-originated cultures, integrifolia (integrifolia ♀ × integrifolia ♂) and pyrifera (pyrifera ♀ × pyrifera ♂) controls and pyrifera ♀ × integrifolia ♂ crosses resulted in healthy progenies, but the reciprocal mate integrifolia ♀ × pyrifera ♂ was inviable. Overall, the viable hybrid and pyrifera crosses showed the highest reproductive success (RS) up to 14 °C and 50 μmol photons m−2 s−1. Towards higher conditions of temperature and light, the hybrid and integrifolia combinations showed better RSs. In sporophyll–originated experiments, we compared the RS in gametophytes from integrifolia and pyrifera sporophytes but also from F1 hybrids cultivated in tanks in northern Chile, under similar temperature and irradiance gradients. The lowest reproductive successes were also detected in integrifolia, although this morph demonstrated a more stable sex ratio throughout thermal gradient. In this case, the hybrid gametophytes were less prolific than in the germplasm trials, but such hybrid combination performed better than the controls at higher temperatures (16–17 °C). Once optimum irradiances and temperature from different studies along the Chilean coast are clustered together against their location, they showed an inverse correlation between optima and a latitudinal gradient for Macrocystis gametogenesis. In such scenario, our crosses cultivated in northern Chile (sporophyll-generated) showed optima towards much higher conditions especially for irradiance. The hybrid gametophytes performed similarly to integrifolia potentially as a result not only from an integrifolia genetic background but also from the cultivation conditions of their sporophyte parents in northern Chile. Implications of these results are discussed in ecological and aquaculture contexts.