Take a look around in the animal world and you will find that, in most organisms, individuals of one sex are larger than the other of the species.
Even though evolutionary biologists have long recognized this discrepancy, called sexual dimorphism, they have struggled for decades to solve a major paradox: How can males and females of one species be of different sizes, given that they share the same genetic blueprints dictating their development and growth?Researchers from the University of Arizona have discovered that the key to unraveling this mystery lies in the early developmental stages during which the sexes begin to grow apart and that females can respond to selection on size almost twice as fast as can males.
"In mammals, the males tend to be larger because there is an advantage in being bigger and stronger when it comes to fighting over who gets the female," explained Craig Stillwell, lead author of the study and a UA Center for Insect Science postdoctoral fellow in the lab of Goggy Davidowitz, an assistant professor of entomology at the UA.
"In most arthropods, on the other hand, we find the opposite: the females are bigger than the males. Think of spiders, for example. In some species, the female can be hundreds of times larger than the male.
"The question we asked was, 'how do females and males come to be different in size?'"
Many biologists have tried to solve this puzzle over time, but when Stillwell and Davidowitz looked at the literature, they realized something was missing in the picture.
"Since there is no difference – at least that we know of – between the male and female genes controlling growth, nobody could figure out why we see what we see in nature: differently sized males and females," said Stillwell.
Scientists have known that growth rates do not differ between female and male caterpillars and thus cannot account for the observed size difference. Rather, the sexual dimorphism observed in the adult animals more likely has to do with differences in the time the two sexes spent as growing larvae. Even in light of that, nearly all research has focused on the adult animals.
"We are the first ones to look at the larvae with this question in mind," Stillwell said.
Stillwell and Davidowitz chose the giant hawk moth (Manduca sexta), a species native to Arizona, as a model organism, mostly because this insect species is well-studied, easily bred in the lab and large enough to allow for ease of handling and measuring.
The researchers followed more than 1,200 caterpillars from the time they hatched, all the way through four molts and until they pupated. They weighed and measured the animals at different times during development and fed the data into a complex statistical model they developed.
For most of their lives as caterpillars, females and males do not appear much different.
"The final larval stage is when it all happens," Stillwell said. "There is a point in the caterpillar's life when an inner clock and environmental cues tell the animal it's time to become an adult. Hormonal changes make them stop feeding and wander around looking for a place to pupate. Within a few hours they develop into a pupa, from which the adult moth will emerge a few weeks later."
Stillwell and Davidowitz discovered that female caterpillars initiate this fundamental change a bit later than the males. By the time the female caterpillars pupate, they are larger, making for larger moths when they emerge.
So where is the advantage in being larger if you're a female insect?
"Biologists think selection favors large females because they can produce more offspring," Stillwell said.
"Another exciting result of this study is that we found a lot more variation in the physiological makeup of the female caterpillars compared to male individuals. Therefore, over generations, the females are able to respond to selective pressures nudging them toward large body size much faster than the males."
Reference: R. Craig Stillwell and Goggy Davidowitz, A developmental perspective on the evolution of sexual size dimorphism of a moth. Proceedings of the Royal Society of London, Series B, published online before print on March 10, 2010.
Daniel Stolte | EurekAlert!
Modern genetic sequencing tools give clearer picture of how corals are related
17.08.2017 | University of Washington
The irresistible fragrance of dying vinegar flies
16.08.2017 | Max-Planck-Institut für chemische Ökologie
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
17.08.2017 | Physics and Astronomy
17.08.2017 | Earth Sciences
17.08.2017 | Physics and Astronomy