Anisogamy is a fundamental form of sexual reproduction where two gametes, or sex cells, of different sizes and often different forms, fuse to create a new organism. This process is widespread across multicellular life, including animals, plants, and fungi. It defines the biological distinction between male and female sexes, with one sex producing smaller gametes and the other producing larger ones.
The Two Types of Gametes
Anisogamy involves two distinct gamete types. The male gamete, known as a microgamete, is typically small and often motile. Sperm cells are common examples of microgametes, produced in large quantities to increase the likelihood of encountering a female gamete. These microgametes generally do not carry significant nutrient reserves.
Conversely, the female gamete, termed a macrogamete, is considerably larger and usually non-motile. Egg cells, or ova, are classic examples of macrogametes, characterized by their substantial size and abundant stored nutrients. While fewer macrogametes are produced compared to microgametes, their larger size and nutrient reserves are designed to support the initial development of the zygote after fertilization.
The Evolutionary Development of Anisogamy
The prevailing scientific theory for the emergence of anisogamy from an ancestral state of isogamy, where all gametes were of uniform size, is disruptive selection. This theory posits that as organisms evolved greater complexity or larger body sizes, there was a selective pressure for the resulting zygote to be larger, providing more resources for early development, favoring the production of larger gametes.
At the same time, a competing selective pressure favored producing many small, motile gametes to maximize the chances of encountering and fertilizing other gametes. This created a trade-off: an individual could either invest its limited reproductive resources into producing a few large, resource-rich gametes that enhance offspring survival, or many small, fast gametes that increase fertilization probability. Intermediate-sized gametes were at a disadvantage, as they were neither large enough to provide substantial resources for zygote survival nor numerous enough to ensure high fertilization rates. This disruptive selection, favoring gamete size extremes and selecting against the middle, led to anisogamy.
Alternative Reproductive Strategies
To fully appreciate anisogamy, it helps to understand its primary alternative: isogamy. Isogamy is a reproductive strategy where the fusing gametes are morphologically identical. In organisms exhibiting isogamy, there is no distinct male or female gamete; instead, they often have different “mating types,” typically designated as “+” and “-” strains, that must fuse for reproduction.
Isogamy is common among most unicellular eukaryotes, such as certain types of algae and fungi. For instance, green algae like Ulothrix produce flagellated isogametes that swim towards each other for fertilization, while Spirogyra, another green alga, produces non-flagellated isogametes that rely on water currents for fusion. This strategy is viable in environments where gamete encounter rates are less constrained by size differences, allowing for a more flexible mating system.
Consequences for Mating and Behavior
The fundamental difference in gamete investment between sexes in anisogamous species shapes mating patterns and behaviors. The sex that produces the larger, resource-rich macrogametes, typically females, makes a greater initial investment in each reproductive event. This higher investment means that macrogametes become a limiting resource, leading to the evolution of choosiness in females, to maximize their investment by selecting mates with advantageous traits.
Conversely, the sex producing numerous, smaller microgametes, typically males, faces selection pressures to maximize their fertilization success through competition for access to mates. This often results in males developing traits for intrasexual competition, such as elaborate displays, weaponry, or aggressive behaviors, to outcompete rivals for mating opportunities. This differential investment and resulting competition and choosiness drive many observable behavioral differences between males and females across the animal kingdom.