|Shifts in sexual systems are one of the key drivers of species diversification. In contrast to
angiosperms, unisexuality prevails in bryophytes. Here, we test the hypotheses that bisexuality
evolved from an ancestral unisexual condition and is a key innovation in liverworts.
We investigate whether shifts in sexual systems influence diversification using hidden state
speciation and extinction analysis (HiSSE). This new method compares the effects of the variable
of interest to the best-fitting latent variable, yielding robust and conservative tests.
We find that the transitions in sexual systems are significantly biased toward unisexuality,
even though bisexuality is coupled with increased diversification. Sexual systems are strongly
conserved deep within the liverwort tree but become much more labile toward the present.
Bisexuality appears to be a key innovation in liverworts. Its effects on diversification are presumably
mediated by the interplay of high fertilization rates, massive spore production and
long-distance dispersal, which may separately or together have facilitated liverwort speciation,
suppressed their extinction, or both. Importantly, shifts in liverwort sexual systems have
the opposite effect when compared to angiosperms, leading to contrasting diversification patterns
between the two groups. The high prevalence of unisexuality among liverworts suggests,
however, a strong selection for sexual dimorphism.|