Trebouxiophyceae are particularly widespread in terrestrial environments and comprise most of the lichen-forming microalgae genera. These symbionts have been frequently considered asexual, however, their life cycles remain largely unknown. We sequenced and analysed the nuclear genome of Trebouxia lynniae, a model Trebouxiophyceae phycobiont, monitored and modelled its colony proliferation and analysed cell population dynamics by using flow cytometry coupled with microscopy and ploidy analysis. The genome inspection unveiled the presence of a “meiosis toolkit”, indicative of sexual reproduction, and the absence of TALE transcription factors related with haplontic life cycles. Moreover, we reveal that T. lynniae possesses a diploid genome, sexual reproduction, and diplontic life cycle. Also, we have demonstrated that its zoospores are gametes, and that meiosis is prezygotic. These discoveries illuminate Trebouxiophyceae ecology and evolution, highlighting the potential adaptive significance of sex in the face of challenging and changing ecological conditions like those faced by lichen symbionts. Moreover, characterizing this terrestrial Chlorophyta's life cycle contributes to shape evolutionary theories that aim to elucidate the path that they took during terrestrialization, suggesting that, as proposed for Streptophyta, it may have been mediated by a life cycle shift.
