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Abstract Sex is a crucial process that has molecular, genetic, cellular, organismal, and population‐level consequences for eukaryotic evolution. Eukaryotic life cycles are composed of alternating haploid and diploid phases but are constrained by the need to accommodate the phenotypes of these different phases. Critical gaps in our understanding of evolutionary drivers of the diversity in algae life cycles include how selection acts to stabilize and change features of the life cycle. Moreover, most eukaryotes are partially clonal, engaging in both sexual and asexual reproduction. Yet, our understanding of the variation in their reproductive systems is largely based on sexual reproduction in animals or angiosperms. The relative balance of sexual versus asexual reproduction not only controls but also is in turn controlled by standing genetic variability, thereby shaping evolutionary trajectories. Thus, we must quantitatively assess the consequences of the variation in life cycles on reproductive systems. Algae are a polyphyletic group spread across many of the major eukaryotic lineages, providing powerful models by which to resolve this knowledge gap. There is, however, an alarming lack of data about the population genetics of most algae and, therefore, the relative frequency of sexual versus asexual processes. For many algae, the occurrence of sexual reproduction is unknown, observations have been lost in overlooked papers, or data on population genetics do not yet exist. This greatly restricts our ability to forecast the consequences of climate change on algal populations inhabiting terrestrial, aquatic, and marine ecosystems. This perspective summarizes our extant knowledge and provides some future directions to pursue broadly across micro‐ and macroalgal species. 

Abstract Sexual systems (i.e., separate vs. combined sexes) vary widely among eukaryotes and influence the evolution of reproductive systems, which shape genetic structure and evolutionary trajectories. In diploid‐dominant angiosperms, combined (i.e., hermaphroditism) and separate sexes are expected to correlate with selfing and outcrossing, respectively. When sex is determined in the haploid phase, selfing is possible even when there are separate sexes. The freshwater red macroalgal genusSheathia(Batrachospermales) displays sexual system variation within and among populations, but no prior data exist on the reproductive systems of these populations. We developed 16 polymorphic microsatellite loci to characterize the reproductive system and genetic structure of threeSheathiaspecies. We observed cross‐amplification of loci across the three targeted species, suggesting these markers may be useful in otherSheathiaspp. We observed variation in monoicy (i.e., hermaphroditism) versus dioicy (i.e., separate sexes) in each species, includingS. americana, which was previously believed to be obligately dioicous. Our data suggest thatS. americanaandS. involutadisplay more variation in their prevailing reproductive modes as compared toS. grandis. Generally, dioicy resulted in greater diversity in contrast to monoicy. We observed strong population structure that is likely driven by uniparental reproduction and limited dispersal; however, there is limited population connectivity that may be facilitated by long‐distance dispersal events. Overall, these data contribute to our knowledge of the relationship between the sexual system, reproductive system, and population genetic structure in haploid‐diploid taxa, thereby informing a broader understanding of the evolution of sex. 

Abstract Temporal population genetic studies have investigated evolutionary processes, but few have characterized reproductive system variation. Yet, temporal sampling may improve our understanding of reproductive system evolution through the assessment of the relative rates of selfing, outcrossing, and clonality. In this study, we focused on the monoicous, haploid‐diploid freshwater red algaBatrachospermum gelatinosum. This species has a perennial, microscopic diploid phase (chantransia) that produces an ephemeral, macroscopic haploid phase (gametophyte). Recent work focusing on single‐time point genotyping suggested high rates of intragametophytic selfing, although there was variation among sites. We expand on this work by genotyping 191 gametophytes sampled from four sites that had reproductive system variation based on single‐snapshot genotyping. For this study, we sampled at multiple time points within and among years. Results from intra‐annual data suggested shifts in gametophytic genotypes throughout the season. We hypothesize that this pattern is likely due to the seasonality of the life cycle and the timing of meiosis among the chantransia. Interannual patterns were characterized by consistent genotypic and genetic composition, indicating stability in the prevailing reproductive system through time. Yet, our study identified limits by which available theoretical predictions and analytical tools can resolve reproductive system variation using haploid data. There is a need to develop new analytical tools to understand the evolution of sex by expanding our ability to characterize the spatiotemporal variation in reproductive systems across diverse life cycles. 

Abstract The freshwater red algaBatrachospermum gelatinosumhas a well‐documented distribution spanning historically glaciated and unglaciated eastern North America. This alga has no known desiccation‐resistant propagule; thus, long‐distance dispersal events are likely rare. We predicted strong genetic structure among drainage basins and admixture among sites within basins. We predicted greater genetic diversity at lower latitude sites because they likely serve as refugia and the origin of northward, post‐Pleistocene range expansion. We used 10 microsatellite loci to investigate genetic diversity from 311 gametophytes from 18 sites in five major drainage basins: South Atlantic Gulf, Mid‐Atlantic, Ohio River, Great Lakes, and Northeast. Our data showed strong genetic partitioning among drainage basins and among sites within basins, yet no isolation by distance was detected. Genetic diversity varied widely among sites and was not strictly related to latitude as predicted. The results fromB. gelatinosumprovide strong support that each stream site contributes to the unique genetic variation within the species, potentially due to limited dispersal and the prevailing reproductive mode of intragametophytic selfing. Simulations of migration suggested post‐Pleistocene dispersal from the Mid‐Atlantic.Batrachospermum gelatinosumpotentially persisted in refugia that were just south of the ice margins rather than in the southernmost part of its range. Research of other taxa with similar ranges could determine whether these results are generally applicable for freshwater red algae. Nevertheless, these results fromB. gelatinosumadd to the growing literature focused on the patterns and genetic consequences of post‐Pleistocene range expansion by eastern North American biota. 

Abstract The relative rates of sexual versus asexual reproduction influence the partitioning of genetic diversity within and among populations. During range expansions, asexual reproduction often facilitates colonization and establishment. The arrival of the green algaAvrainvillea laceratahas caused shifts in habitat structure and community assemblages since its discovery in 1981 offshore of Oʻahu, Hawai‘i. Field observations suggest this species is spreading via vegetative reproduction. To characterize the reproductive system ofA. laceratain Hawai‘i, we developed seven microsatellite loci and genotyped 321 blades collected between 2018 and 2023 from three intertidal sites at Maunalua Bay and ʻEwa Beach. We observed one to four alleles at multiple loci, suggestingA. laceratais tetraploid. Each site was characterized by high genotypic richness (R > 0.8). However, clonal rates were also high, suggesting the vegetative spread ofA. lacerataplays a significant role. The importance of clonal reproduction for the persistence ofA. laceratain Hawai‘i is consistent with the ecological data collected for this species and observations of other abundant macroalgal invaders in Hawai‘i and other regions of the world. These data demonstrate the necessity for implementing appropriate population genetic methods and provide insights into the biology of this alga that will contribute to future studies on effective management strategies incorporating its reproductive system. This study represents one of the few that investigate green algal population genetic patterns and contributes to our understanding of algal reproductive system evolution. 

Abstract Chlainomonas(Chlamydomonadales, Chlorophyta) is one of the four genera of snow algae known to produce annual pink or red blooms in alpine snow. NoChlainomonasspecies have been successfully cultured in the laboratory, but diverse cell types have been observed from many field‐collected samples, from multiple species. The diversity of morphologies suggests these algae have complex life cycles with changes in ploidy. Over 7 years (2017–2023), we observed seasonal blooms dominated by aChlainomonasspecies from late spring through the summer months on a snow‐on‐lake habitat in an alpine basin in the North Cascade Mountains of Washington, USA. The Bagley LakeChlainomonasis distinct from previously reported species based on morphology and sequence data. We observed a similar collection of cell types observed in otherChlainomonasspecies, with the addition of swarming biflagellate cells that emerged from sporangia. We present a life cycle hypothesis for this species that links cell morphologies observed in the field to seasonally available habitat. The progression of cell types suggests cells are undergoing both meiosis and fertilization in the life cycle. Since the life cycle is the most fundamental biological feature of an organism, with direct consequences for evolutionary processes, it is critical to understand how snow algal life cycles will influence their responses to changes in their habitat driven by climate warming. For microbial taxa that live in extreme environments and are difficult to culture, temporal field studies, such as we report here, may be key to creating testable hypotheses for life cycles. 

Abstract Phenology, or seasonal variation in life cycle events, is poorly described for many macroalgal species. We describe the phenology of a non‐native population ofGracilaria vermiculophyllawhose thalli are free‐living or anchored by decorating polychaetes to tube caps. At a site in South Carolina, USA, we sampled 100 thalli approximately every month from January 2014 to January 2015. We assessed the reproductive state and measured thallus size based on wet weight, thallus length, and thallus surface area from herbarium mounts. Because life cycle stage cannot be assigned using morphology, we implemented a PCR assay to determine the life cycle stage—tetrasporophyte, female gametophyte, or male gametophyte—of each thallus. Tetrasporophytes dominated throughout the year, making up 81%–100% of thalli sampled per month. Reproductive tetrasporophytes varied between 0% and 65% of monthly samples and were most common in warm summer months (July through September) when thalli also tended to be larger. The vast majority of the reproductive thalli were worm‐anchored and not fixed to hard substratum via a holdfast. Thus, free‐living thalli can be reproductive and potentially seed new non‐native populations. GivenG. vermiculophyllareproduction seems tied closely to temperature, our work suggests phenology may change with climate‐related changes in seawater temperatures. We also highlight the importance of understanding the natural history of macroalgae to better understand the consequence of range expansions on population dynamics. 

Abstract Reproduction is a fundamental aspect of life that affects all levels of biology, from genomes and development to population dynamics and diversification. The first Tree of Sex database synthesized a vast diversity of reproductive strategies and their intriguing distribution throughout eukaryotes. A decade on, we are reviving this initiative and greatly expanding its scope to provide the most comprehensive integration of knowledge on eukaryotic reproduction to date. In this perspective, we first identify important gaps in our current knowledge of reproductive strategies across eukaryotes. We then highlight a selection of questions that will benefit most from this new Tree of Sex project, including those related to the evolution of sex, modes of sex determination, sex chromosomes, and the consequences of various reproductive strategies. Finally, we outline our vision for the new Tree of Sex database and the consortium that will create it (treeofsex.org). The new database will cover all Eukaryota and include a wide selection of biological traits. It will also incorporate genomic data types that were scarce or non-existent at the time of the first Tree of Sex initiative. The new database will be publicly accessible, stable, and self-sustaining, thus greatly improving the accessibility of reproductive knowledge to researchers across disciplines for years to come. Lastly, the consortium will persist after the database is created to serve as a collaborative framework for research, prioritizing ethical standards in the collection, use, and sharing of reproductive data. The new Tree of Sex consortium is open, and we encourage all who are interested in this topic to join us. 

Abstract The ecological, evolutionary, economic, and cultural importance of algae necessitates a continued integration of phycological research, education, outreach, and engagement. Here, we comment on several topics discussed during a networking workshop—Algae and the Environment—that brought together phycological researchers from a variety of institutions and career stages. We share some of our perspectives on the state of phycology by examining gaps in teaching and research. We identify action areas where we urge the phycological community to prepare itself to embrace the rapidly changing world. We emphasize the need for more trained taxonomists as well as integration with molecular techniques, which may be expensive and complicated but are important. An essential benefit of these integrative studies is the creation of high‐quality algal reference barcoding libraries augmented with morphological, physiological, and ecological data that are important for studies of systematics and crucial for the accuracy of the metabarcoding bioassessment. We highlight different teaching approaches for engaging undergraduate students in algal studies and the importance of algal field courses, forays, and professional phycological societies in supporting the algal training of students, professionals, and citizen scientists.