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Haplo-Diploidy, An Explanation
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Much of this text is not mine, many of the words are
attributed to David H. Headrick, I have merely presented some of his
words in an editorial sense in order to help those that have
difficulty understanding the difference between human and honey bee
reproduction.
There are two types of cells somatic cells and germ cells.
Somatic cells form the various tissues and organ systems of the body,
bone, muscle, liver, heart, brain, etc. Germ cells form the
reproductive tissue from which gametes (sperm and eggs) arise.
In humans, each cell (somatic and germ) has a full
complement of chromosome pairs; this condition is known as diploid.
However, when the germ cells divide to form gametes, each gamete
receives only one set of chromosomes; this condition is called
haploid or having one half the number of chromosomes. When an egg and
sperm unite the diploid condition is restored.
The most common form of reproduction in insects is sexual.
Males and females are present in populations and males pass their
haploid gametes to females for fertilization of the haploid eggs to
produce a diploid zygote; this form is also called biparental
(ie... two parents).
Reproduction can take place in the absence of male gametes,
but is not as common. This is termed Asexual reproduction or
parthenogenesis (Latin for virgin birth). In this form, eggs produced
by females are diploid (they carry the full complement of chromosomes)
and develop into females that are genetic clones of their mothers.
Many species of aphids, some weevils and some Hymenoptera have this
form of reproduction.
A combination of sexual and asexual modes called
haplo-diploidy occurs in four insect orders, the Coleoptera,
Hymenoptera, Thysanoptera, and Hemiptera. Haplo-diploidy occurs in
most of the Hymenoptera or about 100,000 known species. In this form
male gametes are haploid and the eggs produced by females are also
haploid. Mated females have the choice of fertilizing an egg or not.
Unfertilized eggs become male, and thus remain haploid. Fertilized
eggs become female, and are diploid.
Our honey bee is a particular example of this type of
reproduction. Queen bees are fertilized by haploid drones. The queen
then lays fertilized eggs which become female workers. Occasionally
the female will lay unfertilized eggs that ultimately become drones.
Fertilization...
This takes place in most insects in the median oviduct.
Sperm that has been stored in the spermathaeca enters the oviduct by
way of a thin tube, and as the egg passes by the sperm enter the egg
through the egg shell or chorion through a small opening in one end,
known as a micropyle. Fertilization is deemed to take place when the
sperm and egg nucleus unite.
Sex determination
The chromosomes of insects, like most animals occur in
pairs. (There are sixteen pairs in honey bees.) There are two types
of chromosomes, autosomes and sex chromosomes, there is usually a
single pair of sex chromosomes and the genes associated with these
chromosomes account for an individual’s 'maleness' or 'femaleness.'
In humans, the sex chromosomes are structurally different. Females
have two similar chromosomes called X chromosomes (designated as XX),
whereas males have an X chromosome and a smaller Y chromosome (XY).
The presence of the Y chromosome, rather than two X’s, is what
accounts for being male. In insects, however, the males generally
have just one sex chromosome instead of a pair, this condition is
referred to as XO. Female insects are generally XX.
In human male gametes, each sperm has either an X or Y
chromosome; in eggs the sex chromosome is always X. Thus, the gender
of the offspring is determined by whether an X sperm joins with an
egg making an XX female, or a Y sperm joins with an egg producing an
XY male. In insects, the sperm which does not have a sex chromosome
thus creates a male insect (XO).
Written... 18 June 2001, Revised... 12 Aug 2002
