Who you calling Diploid?
By Charles Lindemann
Sperm are the male gamete produced from the germ cell line. The germ cells of the
male are called spermatogonia, which oddly means sperm-eggs. Spermatogonia differentiate
into spermatocytes in the testes, and it is the spermatocytes that divide by a process called
meiosis. Meiosis is a type of cell division that reduces the chromosome number from 46 to 23.
For this reason the sperm cells that are produced after meiosis have only half the full number of
chromosomes. They carry half of the genetic material needed to make a new organism. Since
they only have half the normal number of chromosomes they are referred to as haploid cells as
opposed to the normal body cells that are said to be diploid. Some life forms, such as seaweed
and ferns, spend a large part of their life in the haploid state and actually have a haploid body as
well as a diploid body. Higher animals, including humans, have a life cycle where only a short
time is spent in the haploid state and the haploid organism is limited to single cells we call eggs
and sperm. In this generalized view of things, the sperm and egg are the haploid phase of the
human life cycle. The vast majority of multicellular organisms have a sexual reproductive phase
in their life cycle during which haploid gametes are produced.
In the non-vascular plants (like the seaweed) the gametes can look pretty much alike.
This is called isogamy. The gametes of these organisms usually have two flagella that serve for
swimming and also as arms to grab and stick to each other for fertilization. One of the green
algae that does this is called Chlamydomonas and is a fresh-water single-celled plant.
Chlamydomonas has become the leading experimental model system to study the genetics of
flagella. It is especially good for this because it can re-grow flagella quickly, it can be grown by
the bucket-full in pond water, and it can be mutated extensively to isolate flagella with missing
parts. The National Science Foundation maintains a repository of Chlamydomonas strains at
Duke University as part of the Chlamydomonas Genetics Center. In this resource facility many
useful mutant strains of Chlamydomonas are available for research on the assembly and motility
of the flagellum.
Believe it or not, structural proteins important to human sperm have actually been
identified first in the flagella of this green water plant! This tells us all something about how well
nature has conserved the gamete part of the life cycle. It also shows us how much we share with
the other living things on this earth. Higher animals and plants generally have developed an
uneven size of the eggs and the sperm (heterogamy) with only the sperm retaining a flagellum
and the ability to swim. In the higher plants (herbs, trees and shrubs) the sperm have lost their
flagella and are delivered to the egg through a pollen tube. This hidden sperm feature is called
cryptogamy, which literally means hidden gamete. Higher animals, including man, also have
heterogamy with a big egg and small sperm, but unlike the higher plants the sperm retain the
flagellum for swimming. Human sperm literally swim up the oviduct to meet and fertilize the
egg, which is the other gamete. Most multicellular animals have swimming sperm with flagella,
although a few like the nematode worm Caenorhabditis elegans have lost the flagella for
swimming and move by crawling.
Marine animals, like sea urchins and fish, often release the sperm and eggs right into the
seawater. This often means the sperm must hone in on the eggs from a long distance. The
sperm of sea squirts and sea urchins are known to have a chemical homing mechanism called
chemotaxis that attracts sperm to chemicals released from the egg mass. Recent work published
in the prestigious journal Science suggests the bizarre possibility that such a homing system has
been retained by the sperm even in humans! The chemoattractant molecules that seem to work
for human sperm are odorant molecules. The sperm smell their way to the egg! Fact can
sometimes be stranger than fiction.