Seminar Two
Evolution of sexuality
Gamete types

Why do most sexual organisms have exactly two gamete types, sperm and egg? Why not one, three or four? Let's venture an educated guess: two may be enough to prevent conflict between mitochondria, the cellular organelles which used to be bacteria (see our earlier discussion of mutualisms).

Mitochondria reproduce asexually within our cells and retain some of its own genes. If mitochondria were inherited from both parents of their sexual host, there would be two sets in every cell, each set with its own unique genes, and it might engage in genetic conflict.

In most eukaryotes, however, mitochondria are inherited only maternally, and its genetic uniformity may promote harmony. On the other hand, the presence of exactly two gamete types is not the Holy Grail. Slime molds may have up to thirteen types.

A thing of rare beauty, the plasmodial slime mold shown above was photographed in the moist understory of a Costa Rican rain forest. It is like nothing else on earth. An advancing feeding machine as large as a dinner plate, the thick soup of nuclei called a plasmodium moves at a centimeter per hour, devouring small animals and microorganisms in its path. It is for a short time a single organism with thousands of nuclei in an unbroken matrix which branches in a fractal pattern. It can pass unchanged through a sheet of filter paper.

Then, abruptly, it undergoes a change which advances like a slow wave through it. Some of the nuclei form fruiting bodies called sporangia, which become separate individuals. Shown in the image below, these new individuals form an expanding Lilliputian forest on log and leaf surfaces, even while the brightly colored plasmodium is still moving and feeding a few inches away. The slime mold is changing under our eyes from a single individual into a multitude. The concept of individuality becomes ambiguous.

The life cycle of the slime mold is complete when spores (gametes) from these sporangia fall to the ground, divide, fuse to form zygotes, then create a new multinucleate plasmodium. Up to thirteen different gamete types participate in sexual reproduction. Why and how did such gamete variety evolve, and why is it so rare? Some day we will know.