4.2 A complete loss of sex under extreme warming
Despite that clonal plants usually retain two reproductive modes – sexual seed reproduction and asexual vegetative propagation, a loss of sex has been widely recognized in the evolutionary process of clonal plants (Eckert, 2002). Two prior studies have found that warming reduced the sexual reproduction of P. crispus in the measurement of inflorescence number (Xu et al., 2020; Yan et al., 2021). Although inflorescence can indicate sexual reproduction, fruits or seeds are more directly correlated with plant fitness (Wang et al., 2021b). In the present study, we found a rare complete loss of sex under extreme warming as warming blocked the production of fruit sets for three populations, which corresponds to the second hypothesis. During the experimental process, inflorescence has also been observed. However, no fruit sets formed in the warming treatments. The major pollination mode for P. crispus is hydrophilous (Guo & Huang, 1999). High water temperature likely damaged the pollen viability, which led to the failure in the seed formation (Rang et al., 2011). Santamarı́a & Hootsmans (1998) also found that the seed production of the submerged plant, Ruppia drepanensis was blocked when the water temperature exceeds 30 ℃. Therefore, a probable explanation for the complete loss of sex is that extreme warming disrupted the pollen viability.