The potential for evolutionary responses to cell-lineage selection on growth form and its plasticity in a red seaweed

The role of species thermal plasticity for alien species invasibility in a changing climate: A case study of Lophocladia trichoclados

The Mediterranean Sea provides fertile ground for understanding the complex interplay between invasive species and native habitats, particularly within the context of climate change. This thermal tolerance study reveals the remarkable ability of Lophocladia trichoclados, a red algae species that has proven highly invasive, to adapt to varying temperatures, particularly thriving in colder Mediterranean waters, where it can withstand temperatures as low as 14 °C, a trait not observed in its native habitat. This rapid acclimation, occurring in less than a century, might entail a trade-off with high temperature resistance. Additionally, all sampled populations in the Mediterranean share the same haplotype, suggesting a common origin and the possibility that we might be facing an exceptionally acclimatable and invasive strain. This high degree of acclimatability could determine the future spread capacity in a changing scenario, highlighting the importance of considering both acclimation and adaptation in understanding the expansion of invasive species' ranges.

Fitness effects of somatic mutations accumulating during vegetative growth

The unique life form of plants promotes the accumulation of somatic mutations that can be passed to offspring in the next generation, because the same meristem cells responsible for vegetative growth also generate gametes for sexual reproduction. However, little is known about the consequences of somatic mutation accumulation for offspring fitness. We evaluate the fitness effects of somatic mutations in Mimulus guttatus by comparing progeny from self-pollinations made within the same flower (autogamy) to progeny from self-pollinations made between stems on the same plant (geitonogamy). The effects of somatic mutations are evident from this comparison, as autogamy leads to homozygosity of a proportion of somatic mutations, but progeny from geitonogamy remain heterozygous for mutations unique to each stem. In two different experiments, we find consistent fitness effects of somatic mutations from individual stems. Surprisingly, several progeny groups from autogamous crosses displayed increases in fitness compared to progeny from geitonogamy crosses, likely indicating that beneficial somatic mutations occurred in some stems. These results support the hypothesis that somatic mutations accumulate during vegetative growth, but they are filtered by different forms of selection that occur throughout development, resulting in the culling of expressed deleterious mutations and the retention of beneficial mutations.

Evolution via somatic genetic variation in modular species

Somatic genetic variation (SoGV) may play a consequential yet underappreciated role in long-lived, modular species among plants, animals, and fungi. Recent genomic data identified two levels of genetic heterogeneity, between cell lines and between modules, that are subject to multilevel selection. Because SoGV can transfer into gametes when germlines are sequestered late in ontogeny (plants, algae, and fungi and some basal animals), sexual and asexual processes provide interdependent routes of mutational input and impact the accumulation of genetic load and molecular evolution rates of the integrated asexual/sexual life cycle. Avenues for future research include possible fitness effects of SoGV, the identification and implications of multilevel selection, and modeling of asexual selective sweeps using approaches from tumor evolution.

Somatic Mutation and Evolution in Plants

Somatic mutations are common in plants, and they may accumulate and be passed on to gametes. The determinants of somatic mutation accumulation include the intraorganismal selective effect of mutations, the number of cell divisions that separate the zygote from the formation of gametes, and shoot apical meristem structure and branching. Somatic mutations can promote the evolution of diploidy, polyploidy, sexual recombination, outcrossing, clonality, and separate sexes, and they may contribute genetic variability in many other traits. The amplification of beneficial mutations via intraorganismal selection may relax selection to reduce the genomic mutation rate or to protect the germline in plants. The total rate of somatic mutation, the distribution of selective effects and fates in the plant body, and the degree to which the germline is sheltered from somatic mutations are still poorly understood. Our knowledge can be improved through empirical estimates of mutation rates and effects on cell lineages and whole organisms, such as estimates of the reduction in fitness of progeny produced by within- versus between-flower crosses on the same plant, mutation coalescent studies within the canopy, and incorporation of somatic mutation into theoretical models of plant evolutionary genetics.

Individuality in seaweeds and why we need to care

Documenting the causes and consequences of intraspecific variation forms the foundation of much of evolutionary ecology. In this Perspectives piece, we review the importance of individual variation in ecology and evolution, argue that contemporary phycology often overlooks this foundational biological unit, and highlight how this lack of attention has potentially constrained our understanding of seaweeds. We then provide some suggestions of promising but underrepresented approaches, for instance: conducting more studies and analyses at the level of the individual; designing studies to evaluate heritability and genetic regulation of traits; and measuring associations between individual variation in functional traits and ecological outcomes. We close by highlighting areas of phycological research (e.g., population biology, ecology, aquaculture, climate change management) that could benefit immediately from including a focus on individual variation. Algae, for their part, provide us with a powerful and diverse set of ecological and evolutionary traits to explore these topics. There is much to be discovered.

Somatic Mutation Is a Function of Clone Size and Depth in Orbicella Reef-Building Corals

In modular organisms, the propagation of genetic variability within a clonal unit can alter the scale at which ecological and evolutionary processes operate. Genetic variation within an individual primarily arises through the accretion of somatic mutations over time, leading to genetic mosaicism. Here, we assess the prevalence of intraorganismal genetic variation and potential mechanisms influencing the degree of genetic mosaicism in the reef corals Orbicella franksi and Orbicella annularis. Colonies of both species, encompassing a range of coral sizes and depths, were sampled multiple times and genotyped at the same microsatellite loci to detect intraorganismal genetic variation. Genetic mosaicism was detected in 38% of corals evaluated, and mutation frequency was found to be positively related with clonal size and negatively associated with coral depth. We suggest that larger clones experience a greater number of somatic cell divisions and consequently have an elevated potential to accumulate mutations. Furthermore, corals at shallower depths may be exposed to abiotic conditions such as elevated thermal regimes, which promote increased mutation rates. The results highlight the pervasiveness of intraorganismal genetic variation in reef-building corals and emphasize potential mechanisms generating somatic mutations in modular organisms.

Biochemical evolution in response to intensive harvesting in algae: Evolution of quality and quantity

Evolutionary responses to indirect selection pressures imposed by intensive harvesting are increasingly common. While artificial selection has shown that biochemical components can show rapid and dramatic evolution, it remains unclear as to whether intensive harvesting can inadvertently induce changes in the biochemistry of harvested populations. For applications such as algal culture, many of the desirable bioproducts could evolve in response to harvesting, reducing cost-effectiveness, but experimental tests are lacking. We used an experimental evolution approach where we imposed heavy and light harvesting regimes on multiple lines of an alga of commercial interest for twelve cycles of harvesting and then placed all lines in a common garden regime for four cycles. We have previously shown that lines in a heavy harvesting regime evolve a "live fast" phenotype with higher growth rates relative to light harvesting regimes. Here, we show that algal biochemistry also shows evolutionary responses, although they were temporarily masked by differences in density under the different harvesting regimes. Heavy harvesting regimes, relative to light harvesting regimes, had reduced productivity of desirable bioproducts, particularly fatty acids. We suggest that commercial operators wishing to maximize productivity of desirable bioproducts should maintain mother cultures, kept at higher densities (which tend to select for desirable phenotypes), and periodically restart their intensively harvested cultures to minimize the negative consequences of biochemical evolution. Our study shows that the burgeoning algal culture industry should pay careful attention to the role of evolution in intensively harvested crops as these effects are nontrivial if subtle.

Coalescing red algae exhibit noninvasive, reversible chimerism

Chimerism is produced by the somatic fusion of two or more genetically distinct conspecific individuals. In animals, the main cost of fusion is competition between genetically different cell lineages and the probability of original cell line replacement by more competitive invasive lines, which limits its natural frequency (3%-5%). In red and brown seaweeds, chimerism is widespread (27%-53%), seemingly without the negative outcomes described for animals. The rigidity of cell walls in macroalgae prevents cell motility and invasions. In addition, in moving waters, most somatic fusions involve the holdfast. Histological observations in laboratory-built bicolor macroalgal chimeras indicated that upright axes emerge from the base of plants by proliferation and vertical growth of discrete cell groups that include one or just a few of the cell lineages occurring in the holdfasts. Laboratory experiments showed growth competition between cell lineages, thus explaining lineage segregation during growth along originally chimeric erect axes. Genotyping of the axes showed more heterogeneous tissues basally, but apically more homogeneous ones, generating a vertical gradient of allele abundance and diversity. The few chimeric primary branches produced, eventually became homogenous after repeated branching. Therefore, coalescing macroagae exhibit a unique pattern of post-fusion growth, with the capacity to reverse chimerism. This pattern is significantly different from those in animals and land plants, suggesting chimerism is a biologically heterogeneous concept.

Reproduction at the extremes: pseudovivipary, hybridization and genetic mosaicism in Posidonia australis (Posidoniaceae)

BACKGROUND AND AIMS

Organisms occupying the edges of natural geographical ranges usually survive at the extreme limits of their innate physiological tolerances. Extreme and prolonged fluctuations in environmental conditions, often associated with climate change and exacerbated at species' geographical range edges, are known to trigger alternative responses in reproduction. This study reports the first observations of adventitious inflorescence-derived plantlet formation in the marine angiosperm Posidonia australis, growing at the northern range edge (upper thermal and salinity tolerance) in Shark Bay, Western Australia. These novel plantlets are described and a combination of microsatellite DNA markers and flow cytometry is used to determine their origin.

METHODS

Polymorphic microsatellite DNA markers were used to generate multilocus genotypes to determine the origin of the adventitious inflorescence-derived plantlets. Ploidy and genome size were estimated using flow cytometry.

KEY RESULTS

All adventitious plantlets were genetically identical to the maternal plant and were therefore the product of a novel pseudoviviparous reproductive event. It was found that 87 % of the multilocus genotypes contained three alleles in at least one locus. Ploidy was identical in all sampled plants. The genome size (2 C value) for samples from Shark Bay and from a separate site much further south was not significantly different, implying they are the same ploidy level and ruling out a complete genome duplication (polyploidy).

CONCLUSIONS

Survival at range edges often sees the development of novel responses in the struggle for survival and reproduction. This study documents a physiological response at the trailing edge, whereby reproductive strategy can adapt to fluctuating conditions and suggests that the lower-than-usual water temperature triggered unfertilized inflorescences to 'switch' to growing plantlets that were adventitious clones of their maternal parent. This may have important long-term implications as both genetic and ecological constraints may limit the ability to adapt or range-shift; this seagrass meadow in Shark Bay already has low genetic diversity, no sexual reproduction and no seedling recruitment.

Resolving cryptic species of Bossiella (Corallinales, Rhodophyta) using contemporary and historical DNA

PREMISE OF THE STUDY

Phenotypic plasticity and convergent evolution have long complicated traditional morphological taxonomy. Fortunately, DNA sequences provide an additional basis for comparison, independent of morphology. Most importantly, by obtaining DNA sequences from historical type specimens, we are now able to unequivocally match species names to genetic groups, often with surprising results.

METHODS

We used an integrative taxonomic approach to identify and describe Northeast Pacific pinnately branched species in the red algal coralline genus Bossiella, for which traditional taxonomy recognized only one species, the generitype, Bossiella plumosa. We analyzed DNA sequences from historical type specimens and modern topotype specimens to assign species names and to identify genetic groups that were different and that required new names. Our molecular taxonomic assessment was followed by a detailed morphometric analysis of each species.

KEY RESULTS

Our study of B. plumosa revealed seven pinnately branched Bossiella species. Three species, B. frondescens, B. frondifera, and B. plumosa, were assigned names based on sequences from type specimens. The remaining four species, B. hakaiensis, B. manzae, B. reptans, and B. montereyensis, were described as new to science. In most cases, there was significant overlap of morphological characteristics among species.

CONCLUSIONS

This study underscores the pitfalls of relying upon morpho-anatomy alone to distinguish species and highlights our likely underestimation of species worldwide. Our integrative taxonomic approach can serve as a model for resolving the taxonomy of other plant and algal genera.

Experimental evolution of multicellularity using microbial pseudo-organisms

In a major evolutionary transition to a new level of organization, internal conflicts must be controlled before the transition can truly be successful. One such transition is that from single cells to multicellularity. Conflicts among cells in multicellular organisms can be greatly reduced if they consist of genetically identical clones. However, mutations to cheaters that experience one round of within-individual selection could still be a problem, particularly for certain life cycles. We propose an experimental evolution method to investigate this issue, using micro-organisms to construct multicellular pseudo-organisms, which can be evolved under different artificial life cycles. These experiments can be used to test the importance of various life cycle features in maintaining cooperation. They include structured reproduction, in which small propagule size reduces within-individual genetic variation. They also include structured growth, which increases local relatedness within individual bodies. Our method provides a novel way to test how different life cycles favour cooperation, even for life cycles that do not exist.