PSR - The Paternal Sex Ratio Chromosome
In human beings, almost every cell in the body is diploid
meaning it contains two sets of nearly identical copies of 23 chromosomes, a total of 46
being both female and male. The exception to this are gametes which are eggs or
sperm. They are called haploid for they only
contain one copy of each of the 23 chromosomes, the egg being female and the sperm being
male.
When the female egg is fertilized by the male sperm during sexual reproduction, the two
separate haploid cells fuse together to become one cell called a diploid zygote, meaning
it now has 46 chromosomes with 23 coming from each gamete, which develops into a new
diploid individual. For example, when you were born you inherited 23 chromosomes from your
mother, from the fertilized egg, and 23 chromosomes from your father, from the sperm that
fertilized the egg.
In the order of insects called Hymenoptera,
which is bees, wasps, and ants, it was discovered that the males are haploid and the
females are diploid. This unusual system of sex determination is called haplodiploidy. In
addition, female hymenoptera can selectively fertilize eggs, so that they control the sex
of their offspring.
Some of the hymenoptera are parasitoid which
are parasitic wasps that develop within another insect. Some wasps, such as Encarsia
pergandiella, are autoparasitoids, meaning females develop on homopteran hosts, such as
whiteflies, and males develop as parasitoids of other developing parasitoids.
When parasitoids or autoparasitoids mate, the female collects the sperm in a sperm organ
and then stores it. She then lays the egg and, through a muscular contraction,, releases
the sperm and fertilizes the egg, or closes off the sperm organ and the egg is laid but
not fertilized. If they lay a female egg, they will fertilize it making it diploid. If
they lay a male egg, they do not fertilize it making it haploid.
In autoparasitoids, the females are the primary parasitoids and they develop on the
whitefly, which is the primary host. The female wasp will encounter a whitefly and lay a
female fertilized egg (diploid) on top of it. As the egg grows, it consumes the whitefly
and eats it way inside as it goes through metamorphosis, going from egg to adult. When it
is fully developed, only the shell of the host whitefly is left and the wasp emerges as a
female adult. Then another female wasp will come along and lay an unfertilized male egg
(haploid) egg on top of the adult female wasp (secondary host) that just emerged from the
whitefly, and that egg will kill it in the same manner, which is autoparasitoid. Since
this egg was not fertilized, it now grows into a male haploid wasp.
What is even stranger is that unmated females (virgins) can still lay a male egg as it
does not need to be fertilized. When you put a virgin female on top of the whitefly, they
will not lay a female egg for two reasons: they cannot fertilize it, and somehow they know
that only female eggs are laid on top of the whitefly (the primary host). If a virgin
female does lay an unfertilized egg on the whitefly, it will not develop anyway. Only
female wasps can be produced from being laid on top of whiteflies (the primary host), and
only male wasps can be produced from being laid on top of female wasps of their own kind
(the secondary host).
This study, "Origin of males by genome loss in an autoparasitoid wasp", was an
experiment to explain an unusual finding in an autoparasitoid - males developing as
parasitoids of whiteflies. One possibility was that these females were fertilizing
these eggs as usual, but one set of chromosomes was being lost, and then the egg was
developing as a haploid male. A previous study of one haploidiploid parasitic wasp, Nasonia
vitripennis, had revealed that some males were developing this way - from fertilized
eggs - and that the cause of this was the action of a supernumerary chromosome called the
PSR (paternal sex ratio) chromosome. In N. Vitripennis, it was found that some males were
produced from fertilized eggs. It was discovered that in these fertilized eggs, you had an
egg with two nuclei, one from the female side and one from the male side. However, what
was located in the male nuclei was the extra chromosome called PSR. This extra chromosome
carried by the males was killing the whole male set of chromosomes, leaving only one set
of female chromosomes along with this extra chromosome, which becomes a male haploid wasp.
This PSR chromosome is also called a supernumerary chromosome in the nuclei.
In the study of Encarsia pergandiella, male wasps were also found to come from fertilized
eggs that had been placed on the whitefly host and it was found that, similar to N.
vitripennis, the entire paternal set of chromosomes was being lost. It was thought that
perhaps an extra chromosome, such as the PSR chromosome, was also being carried in them.
However, after further study a supernumerary chromosome was not found.
The casue of genome loss in E. pergandiella is still a mystery. Perhaps the male set
of chromosomes were being killed by another source, such as a virus or transposable
element.
