Chapter 11: The Continuity of Life: Cellular Reproduction

Biology is a field rife with exceptions to the norm. You have learned that mitosis is the division of the nucleus, and that it is usually followed by cytokinesis, or cell division. But there’s a reason that word "usually" is there. Sometimes mitosis happens, but cytokinesis is nowhere to be seen.

What happens when this occurs? Because there are two nuclei were once there was one, each of these cells now has two nuclei. And it doesn’t stop there. Cells in which this occurs usually undergo several bouts of mitosis without cytokinesis, so they end up having numerous—sometimes thousands—of nuclei per cell, a condition referred to as being multinucleate.

In humans, there are a few types of tissues where this happens. The best known are cardiac and skeletal muscle tissue. But as a general rule, you can think of almost all of your cells as being regular cells with a single nucleus.

However, among fungi and algae, there are entire groups of species where the adult organism is one huge multinucleate cell. These are some of the most fascinating organisms, and many are beautiful as well. For example, there is an alga that is common in shallow tropical and subtropical waters called Caulerpa. This alga is surprisingly complex in structure, with different species having different forms, from grape-like clusters to feather-like fronds. It is hard to believe that these elaborate structures are composed of a single, large, multinucleate cell.

Why should multinucleate algae have evolved? One possibility is that they are capable of very rapid growth, which all these species seem to be capable of. In fact, one species, Caulerpa taxifola, has become a pest in several places where it is not native but has become introduced, most likely by escaping from aquaria. Large areas of the Mediterranean and of Southern California have become infested.

Fortunately, a solution may be at hand. Certain sea slugs have become specialized to feed on these multinucleate algae. If they can be introduced into the areas where Caulerpa has become a problem, they may help to control the outbreak. However, introducing yet another foreign species could potentially create other, as yet unforeseen problems, so scientists are debating whether to release these sea slugs or not. For an excellent summary of the Caulerpa problem, including the question of releasing these sea slugs, visit this site at the University of Salzburg.