Celebrating Research Devoted to Seed‐Free Land Plants

Abstract

This special issue celebrates the unique diversity of seed‐free land plants (SFLP) and in particular the contribution of Jochen Heinrichs (1969–2018), who dedicated his research to the study of the evolution and taxonomy of these plants (Krings et al., 2018). Given the remarkable diversity of flowering plants, exceeding more than 350 000 extant species, SFLP tend to be often ignored despite their long successful phylogenetic history and their important contribution to terrestrial environments. Seed‐free land plants comprise five extant lineages that share the plesiomorphic trait of spores instead of seeds as their core dispersal unit. Land plants, and thus SFLP, originated approximately 500 million years and dominated terrestrial life for more than 100 million years until the first appearance of seed plants in the late Devonian (Morris et al., 2018). Extant SFLP include the three lineages with dominant gametophytes and sporophytes depended on those— hornworts, liverworts, and mosses—and the two lineages with dominant sporophytes and mostly independent gametophytes—lycophytes and ferns. The latter two lineages share with seed plants not only the common ancestor but also major phenotypic structures, such as vascular tissue and branched sporophytes. The first three lineages are distinct by the absence of vascular tissue and the presence of unbranched sporophytes but show distinct characteristics. Until now, the relationships among these three lineages have been controversial, despite most recently the usage of genomic evidence supporting either a monophylum including all three lineages (=bryophytes) or a monophylum including liverworts and mosses (=setophytes) with the hornworts sister to all land plants or sister to vascular plants (Puttick et al., 2018; Sousa et al., 2019). The authors of the nine contributions of this special issue aimed to highlight different aspects of the evolution and biology of these remarkable plants. The topics were selected not only to reflect current major trends in the study of these plants but especially to highlight research questions that were in the focus of Jochen Heinrichs’ research passions. He devoted the majority of his research carrier, and thus of his life, to the study of the plants he loved and admired. As a consequence of the limitations of a special issue we highlight three aspects. The first aspect is focused on the Mesozoic and Cenozoic fossil record of SFLP. Although Jochen Heinrichs was trained as a neobotanist, he made major contributions to the study of the paleobotany of liverworts and other SFLP by contributing his outstanding knowledge on the extant diversity to research collaboration with leading paleobotanists such as Alexander Schmidt and Michael Krings. The second aspect is focused on the biology and taxonomy of liverworts. Jochen Heinrichs studied these plants for many years and contributed to the revolution in our understanding of their evolution by using the molecular systematics toolkit. The third aspect is focused on the improved ability to study the evolution of genomes to answer major questions, such as the maintenance of species identity despite introgression and the role of processes shaping the genomic disparity in SFLP. The special issue contains the description of one extant liverwort, Lejeunea heinrichsii (Lee et al., 2019), and one extinct fern genus, Heinrichsia (Regalado et al., 2019), that are both new to science and named in memory of Jochen Heinrichs. The first set consists of three contributions devoted to the study of the fossil record of SFLP during the Mesozoic and Cenozoic (Mamonto & Ignatov, 2019; Regalado et al., 2019; Sadowski et al., 2019). In recent years, major advances have been achieved in our understanding of the evolutionary history of these plants especially, but not exclusively, by studying amber inclusions. The often exquisite preservation of phenotypic characters in amber‐enclosed organisms provides crucial information on plant morphology and ecological conditions during the Cretaceous and Cenozoic. Two studies report specific advances in the study of amber fossils. The study by Sadowski et al. (2019) provides for the first time a critical review of the fern diversity recovered in Baltic amber, whereas the study by Regalado et al. (2019) added a further critical piece to resolve the remarkable fern diversity preserved in Cretaceous Burmese amber inclusions. The study on the bryophyte of the Mesozoic of Transbaikalia (Mamonto & Ignatov, 2019) expands the exploration of the fossil record to non‐amber fossils of the early Cretaceous and late Jurassic, including the vegetative body of thalloid liverworts and capsules of derived mosses. The second set consists of three studies reporting progress in our understanding of liverworts (Lee et al., 2019; Pressel & Duckett, 2019; Yu et al., 2019). The biology and taxonomy of these fantastic plants were some of the main research passions of Jochen Heinrichs. The study by Lee et al. (2019) used a molecular phylogenetic approach to clarify the identity of a Lejeunea accession collected from Fiji. The results provide further support to the argument that the taxonomy and biogeography of derived liverworts require studies incorporating comprehensive taxon sampling. The study of Pressel & Duckett (2019) addresses the efficiency of the reproductive systems of the liverwort Marchantia. Although the efficiency of sexual reproduction is of special interest to explain biological patterns, such as their distribution range, very little information has been accumulated on this topic until now. The third paper focusing on liverworts reports results of the first comparative study exploring the evolution of plastid genomes in this lineage of