Tatsuya Togashi

Spatial coexistence of phytoplankton species in ecological timescale

The species diversity of phytoplankton is usually very high in wild aquatic systems, as seen in the paradox of plankton. Coexistence of many competitive phytoplankton species is extremely common in nature. However, experiments and mathematical theories show that interspecific competition often leads to the extinction of most inferior species. Here, we present a lattice version of a multi-species Lotka–Volterra competition model to demonstrate the importance of local interaction. Its mathematical equilibrium is the exclusion of all but one superior species. However, temporal coexistence of many competitive species is possible in an ecological time scale if interactions are local instead of global. This implies that the time scale is elongated many orders when interactions are local. Extremely high species diversity of phytoplankton in aquatic systems may be maintained by spatial coexistence in an ecological time scale.

The evolution of anisogamy: A fundamental phenomenon underlying sexual selection

Introduction Paul Alan Cox 1. The origin and maintenance of two sexes (anisogamy), and their gamete sizes by gamete competition Geoff A. Parker 2. The evolutionary instability of isogamy Hiroyuki Matsuda and Peter A. Abrams 3. Contact, not conflict, causes the evolution of anisogamy Joan Roughgarden and Priya Iyer 4. Nucleo-cytoplasmic conflict and the evolution of gamete dimorphism Rolf F. Hoekstra 5. Adaptive significance of egg size variation of aquatic organisms in relation to mesoscale features of aquatic environments Kinya Nishimura and Noboru Hoshino 6. Gamete encounters David B. Dusenbery 7. Evolution of anisogamy and related phenomena in marine green algae Tatsuya Togashi and John L. Bartelt Index.

Evolutionary trajectories explain the diversified evolution of isogamy and anisogamy in marine green algae

The evolution of anisogamy (the production of gametes of different size) is the first step in the establishment of sexual dimorphism, and it is a fundamental phenomenon underlying sexual selection. It is believed that anisogamy originated from isogamy (production of gametes of equal size), which is considered by most theorists to be the ancestral condition. Although nearly all plant and animal species are anisogamous, extant species of marine green algae exhibit a diversity of mating systems including both isogamy and anisogamy. Isogamy in marine green algae is of two forms: isogamy with extremely small gametes and isogamy with larger gametes. Based on disruptive selection for fertilization success and zygote survival (theory of Parker, Baker, and Smith), we explored how environmental changes can contribute to the evolution of such complex mating systems by analyzing the stochastic process in the invasion simulations of populations of differing gamete sizes. We find that both forms of isogamy can evolve from other isogamous ancestors through anisogamy. The resulting dimensionless analysis accounts for the evolutionary stability of all types of mating systems in marine green algae, even in the same environment. These results imply that evolutionary trajectories as well as the optimality of gametes/zygotes played an important role in the evolution of gamete size.

The paradox of enrichment in phytoplankton by induced competitive interactions

The biodiversity loss of phytoplankton with eutrophication has been reported in many aquatic ecosystems, e.g., water pollution and red tides. This phenomenon seems similar, but different from the paradox of enrichment via trophic interactions, e.g., predator-prey systems. We here propose the paradox of enrichment by induced competitive interactions using multiple contact process (a lattice Lotka-Volterra competition model). Simulation results demonstrate how eutrophication invokes more competitions in a competitive ecosystem resulting in the loss of phytoplankton diversity in ecological time. The paradox is enhanced under local interactions, indicating that the limited dispersal of phytoplankton reduces interspecific competition greatly. Thus, the paradox of enrichment appears when eutrophication destroys an ecosystem either by elevated interspecific competition within a trophic level and/or destabilization by trophic interactions. Unless eutrophication due to human activities is ceased, the worlds aquatic ecosystems will be at risk.

Equal Sex Ratios of a Marine Green Alga, Bryopsis plumosa

By finding some important culture conditions as below, we succeeded in experimentally controlling the whole life history of a dioecious marine green alga, Bryopsis plumosa (Hudson) C. Agardh. In this study, we focused on the primary and secondary sex ratios (i.e. at inception and maturity) using these culture techniques. Gametogenesis was induced by culturing haploid gametophytes with Provasolis enriched seawater (PES) medium under a 14:10 h light:dark cycle at 14 degrees C. Formed zygotes grew into diploid sporophytes, which were cultured for 3 months with PES medium under a 14:10 h light:nbsp;dark cycle at 18 degrees C. Then they were transferred into Schreiber medium and cultured under a 10:14 h light:dark cycle at 22 degrees C. Within 1 week, zoosporogenesis was observed. Zoospores were released within a couple of days. Each zoospore soon germinated and grew into a unisexual gametophyte. The primary sex ratio was examined in gametophytes that originated from a single sporophyte. The secondary sex ratio was studied in the field. Both were estimated as 1:1. Synchronized meiotic cell divisions might occur during zoosporogenesis dividing each sex-determining factor evenly among zoospores. Given the equal sex ratio at maturity, there seems to be no environmental factor that differentially affects the survival of male or female gametophytes in nature.

Ammonium supply mode and the competitive interaction between the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda

We performed competition culture experiments using the bloom-forming cyanobacterium Microcystis novacekii (Kom.) Comp. and the green alga Scenedesmus quadricauda, (Turpin) Brebisson to study the effects of nutrient supply modes on the competition of phytoplankton. The cells were grown in continuous cultures under nitrogen limitation. Ammonium, as the sole nitrogen source, was supplied either continuously or in pulses as one pulse per two days, and the effects of the mode of nutrient supply were examined. While both the species grew well in monoculture, the growth of one species was suppressed by the presence of the other species in the mixed cultures, indicating the occurrence of competition for the supplied ammonium. The competitive outcomes depended on the mode of ammonium supply: under continuous supply, though both the species tended to grow together, the growth of S. quadricauda was suppressed by the presence of M. novacekii, compared to that in the monoculture. Under the pulsed supply, M. novacekii tended to be suppressed by S. quadricauda. This suggests that M. novacekii is a species that grows better in low ammonium supply, while S. quadricauda is a species that grows better in high ammonium supply. The temporally high concentration of ammonium immediately after the pulsed nutrient supply provided an opportunity for S. quadricauda to outgrow or almost exclude M. novacekii.

Power Law for Extinction Process in Multiple Contact Process

We deal with a multiple contact process, that is a modification of the contact process. This system contains N kinds of species ( N = 10) on a finite-sized square lattice. Simulations are carried out using two different methods: local and global interactions. It is found that the waiting time to the extinction of the first species follows a power law. Moreover, we find that local interaction promotes the coexistence of multiple species.

Effects of gamete behavior and density on fertilization success in marine green algae: insights from three-dimensional numerical simulations

We developed a numerical simulation of mating experiment to study effects of phototactic gamete behavior and density on fertilization success, using the C++ programming language, and pseudo-parallelization methods with input parameters based on experimental data. In our experiments, we found that gametes with positive phototaxis are favored, particularly in shallow water, because they can search for potential mates on the two-dimensional (2-D) water surface rather than randomly in three dimensions. We also found evidence that sperm (male gametes) limitation might not be the dominant selective force in the evolution of isogamous or slightly anisogamous marine green algae because almost all of female gametes can be fertilized on the 2-D water surface meaning they might not be under sperm limited conditions. Gamete density also appears to affect mating success seriously. These findings were produced by some technical progress made recently to rapidly and correctly count the numbers of zygotes formed calculating the locations of huge numbers of male and female gametes in the test tank. Both gamete behavior and density might be determined by environmental conditions of habitat, particularly the depth of water.

A geometrical approach explains Lake Ball (Marimo) formations in the green alga, Aegagropila linnaei

An extremely rare alga, Aegagropila linnaei, is known for its beautiful spherical filamentous aggregations called Lake Ball (Marimo). It has long been a mystery in biology as to why this species forms 3D ball-like aggregations. This alga also forms two-dimensional mat-like aggregations. Here we show that forming ball-like aggregations is an adaptive strategy to increase biomass in the extremely limited environments suitable for growth of this alga. We estimate the maximum biomass attained by ball colonies and compare it to that attained by mat colonies. As a result, a ball colony can become larger in areal biomass than the mat colony. In the two large ball colonies studied so far, they actually have larger biomasses than the mat colonies. The uniqueness of Lake Balls in nature seems to be due to the rarity of such environmental conditions. This implies that the conservation of this alga is difficult, but important.

Lattice Population and Optimality of Sex Ratio: Effect of Sterile Male

To know the optimality under given conditions is one of most important problems in various fields. In the present paper, we focus on the optimality of sex ratio i¾?in animals. So far, the optimal sex ratio has been obtained by evolutionarily stable strategy (ESS) in most cases. Recently, however, our coauthors have presented a lattice model of mating population to explain the optimality not by ESS but by the sustainability. They mainly studied the symmetrical case between male and female. In the present paper, we deal with asymmetrical cases: there are sterile male. Our results relatively well explain the evolution of animal sex ratio.