Learning from Nature: Reproductive Strategies for Adapting to Environmental Changes

Midori Matsumoto (Associate Professor, Department of Biosciences and Informatics)

Life on Earth began 3.6 billion years ago. It is believed that over 10 million species of organisms currently coexist on Earth, adapting to various environments. Organisms repeatedly compete among species and individuals to adapt to the changing global environment. To win this competition and raise the next generation, organisms employ what is known as a "reproductive strategy."

Before discussing this reproductive strategy, let's talk about "sex." The Japanese dictionaryKojiendefines "sex" as "the distinction between male and female," but in biological terms, it means "the mixing of genetic information (genomes) from males and females in reproduction." There are two modes of reproduction for creating the next generation: asexual reproduction, which does not involve the mixing of genomes, and sexual reproduction, which does. Asexual reproduction uses methods such as regeneration, budding, and parthenogenesis, creating the next generation's genome by replicating and copying it through somatic cell division.

On the other hand, sexual reproduction involves the mixing of genomes from the germ cells of two individuals through processes like conjugation and fertilization, as well as genomic recombination through meiosis, which is specific to germ cells. Comparing the two reproductive modes, sexual reproduction, which requires forming specialized germ cells and finding a partner, takes more time and energy than asexual reproduction. However, in terms of the diversity of the resulting offspring, sexual reproduction—where recombination and genome mixing occur—surpasses asexual reproduction, which produces uniform copies. This increases the potential to acquire adaptability to the environment. Thus, both reproductive modes have their pros and cons (Figure 1).

Some organisms get the "best of both worlds" by switching between these two reproductive modes depending on environmental conditions such as temperature and day length, even within the same species or individual. A prime example is the planarian, a type of flatworm. In nature, some planarians reproduce asexually by growing to a certain size, undergoing fission, and then regenerating. Others reproduce sexually by developing reproductive organs such as ovaries and testes, pairing up, and mating. Furthermore, some can switch between these two modes depending on environmental conditions like temperature and season (Figure 2).

I am focusing on the mechanism of this switch in reproductive mode. Planarians switch their reproductive mode in response to stimuli on an annual scale, such as seasonal changes. To investigate this system in detail at the molecular level, we have developed our own experimental sexualization system to artificially induce a switch from an asexual to a sexual state in individuals. By applying past findings and feeding asexual individuals with sexual ones daily, we were able to induce a sexual state with 100% efficiency after one month. This has enabled us to advance our research on the switch in reproductive mode, including topics like germ cell differentiation, the determination of reproductive mode, and the relationship between reproductive mode and lifespan.

Nature is full of phenomena that exceed our imagination. The motto of our laboratory is "Study nature, not books." These are the words of Professor Louis Agassiz, a zoologist at Harvard University. It means that instead of being satisfied with what you read in books, you should experience nature firsthand to understand its grand mechanisms.

This quote is housed in the library of the Marine Biological Laboratory in Woods Hole, one of the world's largest marine biological stations (Figure 3). It's quite a self-admonishing message, isn't it?