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Eggs or live young: a trips'll try both.

Eggs or live young: A thrips'll try both

Chickens lay eggs; humans make babies. And the rule has always been that an animal species must settle on one reproductive mode or the other. Not so for the tiny, tree-living thrips, the first known animal that can switch between laying eggs and bearing live young, says biologist Bernard J. Crespi, who discovered the thrips' reproductive versatility while at the University of Michigan's Museum of Zoology in Ann Arbor.

Crespi says his finding will give scientists the first animal with which to test theories of the ecological and social factors important in the evolution of the two kinds of reproduction.

Crespi first demonstrated that the 5-millimeter Elaphrothrips tuberculatus uses two separate reproductive modes by dissecting the ovaries of more than 738 pregnant females and finding them specialized for either egg-laying or bearing live young. Then, to find out whether individual females can switch between the two modes, he watched summer-generation females, which live long enough to reproduce several times, breed on dead oak leaves inside nylon bags. In both 1986 and 1987, about one-third used both reproductive modes, he reports in the Jan. 26 NATURE.

Crespi also noticed that all 1,051 larvae from the 42 egg-laying females were female and that all 202 non-egg larvae from 32 live-young-producing females were male. The male-bearing females produced fewer offspring than those laying eggs, but proportionally more of their young survived.

Crespi went on to look at other thrips species. "The most exciting thing is that [this reproductive versatility occurs] in not just one species but a dozen," he says.

Before Crespi's discovery, scientists had assumed all E. tuberculatus lay eggs, but they knew some females became pregnant without producing eggs. Crespi sexed newborns from eggs and realized they were all female. "We knew the males existed but had no idea where they were coming from," he says.

Crespi says he had been studying this species of thrips for more than a year before he figured it out. Upon dissecting nonegg-laying females with curiously large abdomens, he found that "lo and behold, [they were] full of larvae."
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Publication:Science News
Date:Feb 18, 1989
Words:353
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