Flies that cannot make a type of pheromone are "sexually irresistible¹" to other flies - regardless of gender².

苍蝇自身并不能制造出一种“不可抗拒的性吸引”外激素——排除性别因素。

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The pheromone-secreting cells expressed a fluorescent protein

Scientists bred fruitflies that were unable to secrete⁴（隐秘，隐匿） hydrocarbon⁵（碳氢化合物） signalling chemicals.

They report in the journal Nature that hydrocarbon-free male flies attempt to copulate（交配，交尾） with each other.

In fact, when these chemical signals are removed, flies are completely unable to recognise gender or species barriers.

Joel Levine, a neuroscientist from the University of Toronto at Mississauga in Canada, led the team that carried out the research.

"The big question we wanted to ask was: how does a fly recognise another fly?" he told BBC News.

His team bred their experimental flies using a transgene.

This is a gene⁶ which is inserted into the fly genome and contains a specific set of instructions - in this case to kill the cells that secrete hydrocarbons⁷.

The scientists marked the target cells, called oenocytes, with a fluorescent dye（荧光染料）. This meant they could easily verify that all of the cells had been destroyed.

They were then able to find out how other flies responded to the "manipulated" or hydrocarbon-free flies.

"Normal males found flies without the cells more attractive - whether they were male or female," said Dr Levine.

"Males also prefer females that don't have the cells."

Nicolas Gompel, a biologist from the Institute of Developmental Biology in Marseille, France, commented: "This result contradicts（反驳，矛盾） the widespread belief that pheromones（信息素） are required to initiate⁸（开始，创始） courtship."

Dr Gompel is an expert in insect evolution and behaviour and was not involved in this study.

"[It] suggests that other sensory⁹ inputs¹⁰ such as vision may be a trigger for mating," he added.

Chemical control

The scientists found that flies without the hydrocarbon-secreting cells were also attractive to other species of fruitfly.

"But we found that we could resurrect（复活） the species recognition using just one compound," explained Dr Levine.

"This would stave off（避开） advances from other species."

The manipulated flies provided a sort of blank canvas（帆布） to allow the scientists to test the role played by each chemical - and how the chemical signals interacted.

"We found that one compound - one that males transfer on to females when they copulate - kept other males away," said Dr Levine.

"It's the male's way of sort of protecting his investment."

But females were able to offset¹¹ this repellent（排斥力） by producing another single compound. When mixed with the male chemical, this produced a type of fly aphrodisiac（引起性欲的）.

"So she can make her own stuff to reduce the effect of his stuff," said Dr Levine.

"The effect of a compound very much depends on what other compounds are around."

The researchers think that, without these chemicals, a fly would be attractive to every other fly.

The hydrocarbons, they say "superimpose（重叠） sexual identity" on to each fly.