Insect defies nature
By VANESSA WILLIAMS, science reporter
13jul01 Herald Sun
THE bizarre sex secrets of a girl-power insect that lives in Victorian back yards have
been revealed.
Monash University researcher Dr Andrew Weeks discovered the unique mite is made up of
females with only one set of chromosomes, instead of two, like almost all other life.
Males are so rare that the mother mites have dropped sex for virgin births – laying
unfertilised eggs that hatch live young.
The discovery, published in Science magazine, questions whether sex between males and
females is the best way for animals to reproduce.
Dr Andrew Weeks said current wisdom believed sexual reproduction (where egg and sperm
combine to produce offspring with two sets of chromosomes, one from each parent) became
the dominant way to produce babies because it was a better way for species to evolve and
survive.
The false spider mite, Brevipalpus phoenicis is a tiny animal related to the spider. The
eight-legged creature is only 0.25mm long and can only be studied under a microscope.
It is a crop pest in Brazil and has been known to attack at least one species of
Australian native plant. One infected tree can carry about 50,000 mites.
Scientists knew the mite had only two chromosomes, but were not sure if both came from the
mother or one each from the mother and father.
Dr Weeks said that the clue to the all-female species lay both in a mystery bacteria that
lived in the mite's eggs and how related mite species reproduced.
The mite's relatives belong to an unusual group of insects who sexually reproduce but
whose males have only one set of chromosomes, whereas females have two sets.
Dr Weeks said about 17 male-only haploid species had been discovered but never a female
with only one set of chromosomes.
Other all-female species include some flies, reptiles, fish and wasps, but they reproduced
by cloning themselves so their daughters still had two sets of chromosomes.
"The current theory is that diploidy (having two sets of chromosomes) is the dominant
state because this is a way to protect from mutation," Dr Weeks said.
"If you have a mutation on one of your chromosomes, you've still got the other copy
of the chromosome to compensate," he said.
"But if you are haploid (one set of chromosomes), if you have a bad mutation, you
just die.
"Maybe instead of diploids being necessarily a better way of reproducing, a long time
ago sexual reproduction happened and it's been too hard to go back."
The ancestor of the false spider mite was probably haplo-diploid (haploid male; diploid
female) but a long time ago a bacteria infected the eggs of a diploid female and somehow
caused all the male embryos to turn female.
When Dr Weeks gave the female mites antibiotics, their offspring reverted back to the
normal half-male, half-female split.
Dr Weeks said lab-created males and females did copulate. "But we don't know if they
even transmit sperm or whether the sperm is functional."END OF REPORT
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