![]() The HD-TALE TFs KNOX/BELL have been shown to control gamete fusion, which is mediated through the flagella, in the unicellular alga Chlamydomonas reinhardtii (Lee et al. To further our understanding of how the plant male germ line is controlled we examined genes putatively involved in this process, which is directly linked to sexual reproduction. In well-studied flowering plant models such as Arabidopsis thaliana the male gametophyte has been secondarily reduced, characterized by a loss of flagella and mitosis occurring inside the microscopic pollen. Flagellated sperm cells are an ancestral character of eukaryotes (Stewart and Mattox ( 1975), Mitchell 2007) that has been secondarily lost in the Zygnematales (belonging to the streptophytic algae), the conifers and Gnetales, and the flowering plants. This is enacted, e.g., by pollen in flowering plants and conifers, while most other plant lineages rely on flagellated male gametes (Renzaglia and Garbary 2001). A prerequisite for successful alternation of generations is the male gamete reaching the female gamete. In most eukaryotic lineages the male gametes bear flagella and are motile. Intriguingly, the haploid-to-diploid transition is an ancestral eukaryotic feature that is controlled by homeodomain (HD) transcription factors (TF) of the TALE (three amino acid loop extension) class (Joo et al. Despite these differences, eukaryotic life cycles are united by the diploid zygote arising from the fusion of two haploid gametes. This “alternation of generations” (Hofmeister ( 1979) is in contrast to e.g., diplontic multicellular animals (Metazoa) in which only the diploid phase is multicellular, or haplontic streptophytic algae (sister lineage to land plants), in which only the haploid phase is multicellular. In the process of sexual reproduction the haploid gametophyte develops sperm and egg cells, while the diploid sporophyte undergoes meiosis, yielding spores from which in turn the gametophyte develops. ![]() Taken together, we find that involvement of both regulators in sexual reproduction is conserved since the earliest divergence of land plants.Īll land plants (Embryophyta) possess a haplodiplontic life cycle, i.e., both the haploid and diploid generation (phase) divide mitotically and are hence multicellular. polymorpha and found that in Mp hag1 the development of gametangiophores is impaired. We analyzed HAG1 in the dioecious liverwort M. Pp hag1 additionally shows arrested male and impaired female gametangia development. In both mutants, due to lack of fertile spermatozoids, fertilization and hence the switch to the diploid generation do not occur. We selected two orthologs, SWI3a/b and HAG1, and analyzed loss-of-function mutants in the moss P. We determined candidates by selecting single copy orthologs that are involved in transcriptional control, and of which flowering plant mutants show defects during sexual reproduction, with a focus on the under-studied male germ line. Moreover, embryo lethal mutants can be grown and vegetatively propagated due to the dominance of the bryophyte gametophytic generation. Bryophytes are sister to vascular plants and hence allow evolutionary inferences. Here we set out to determine gene function and conservation via studies in bryophytes. Analyses of such regulators in flowering plants are difficult due to the highly reduced gametophytic generation, and the fact that loss of function of such genes often is embryo lethal in homozygous plants. Several key regulators that control the bauplan of either generation are already known. The switch between these phases was coined alternation of generations. With the water-to-land transition, land plants evolved a peculiar haplodiplontic life cycle in which both the haploid gametophyte and the diploid sporophyte are multicellular. ![]() Bryophytes as models to study the male germ line: loss-of-function mutants of epigenetic regulators HAG1 and SWI3a/b demonstrate conserved function in sexual reproduction. ![]()
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