Systematics and the Origin of Species On Ernst Mayr's 100th Anniversary (2005) / Chapter Skim
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3 Inter-Locus Antagonistic Coevolution as an Engine of Speciation: Assessment with Hemiclonal Analysis--WILLIAM R. RICE, JODELL E. LINDER, URBAN FRIBERG, TIMOTHY A. LEW, EDWARD H. MORROW, AND ANDREW D. STEWART
3 Inter-Locus Antagonistic Coevolution as an Engine of Speciation: Assessment with Hemiclonal Analysis--WILLIAM R. RICE, JODELL E. LINDER, URBAN FRIBERG, TIMOTHY A. LEW, EDWARD H. MORROW, AND ANDREW D. STEWART
Pages 24-45
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From page 24... ...
Here, we focus on the hy pothesis that inter-locus antagonistic coevolution drives rapid ge netic divergence among allopatric populations and thereby acts as an important "engine" of speciation. We assert that only data from studies of experimental evolution, rather than descriptive patterns of molecular evolution, can provide definitive evidence for this hypothesis.
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From page 25... ...
, there is no general consensus regarding the relative importance of alternative evolutionary processes that drive the specific genetic divergence that leads to reproductive isolation. Because allopatry necessitates that populations be physically separated, there can be no direct selection for reproductive isolation and therefore it must develop as a pleiotropic byproduct of the genetic differences that accrue due to the independent evolution of populations.
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From page 26... ...
can potentially drive rapid genetic divergence among allopatric populations because the antagonistic cycle of adaptation and counteradaptation maintains a persistent lag-load at each interacting locus and thereby drives perpetual evolutionary change. In this article, we focus on intergenomic conflict because two of its forms are predicted to contribute to reproductive isolation by causing genetic divergence among allopatric populations (Parker and Partridge, 1998; Rice, 1996, 1998; Rice and Holland, 1997)
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. The arms races between male offense and defense, and both of these processes with female resistance, would be expected to occur independently in allopatric populations and drive rapid genetic divergence among them (Fig.
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From page 28... ...
This alternative explanation for the same molecular data illustrates the problem with using descriptive studies of molecular evolution to test the hypothesis that inter-locus arms races are driving genetic divergence among populations. Because the data are correlative and do not directly measure selection, descriptive studies of molecular evolution can provide supporting evidence for inter-locus arms races but cannot provide definitive evidence.
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From page 29... ...
Because the flies are assayed in the outbred state and in an environment to which they have adapted at large size for hundreds of generations, we were able to obtain direct experimental evidence for the inter-locus arms race between the sexes that can drive the genetic divergence that leads to reproductive isolation. The technique that we describe can be applied only to laboratory populations, but we assert that laboratory populations can be constructed in such a way that the study of their evolution provides an essential complement to studies of natural populations.
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When laboratory island populations are analyzed, strong inference is possible for all forms of genetic and phenotypic variation, and its adaptive significance, because the organisms are measured in the same environment to which they have adapted for hundreds of generations.
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. The utility of using laboratory populations to study evolution depends upon how much their evolution is regulated by the same principles that control the evolution of natural populations.
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Because the LHM population has such a long history of adaptation to a prescribed laboratory environment, we are able to measure both its genetic and fitness variation in the environment to which it is adapted, and thereby assess antagonistic coevolution within and between the sexes. Because the model organism is D
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From page 33... ...
These females carry a random Y chromosome from the LHM base population, an attached X (both X chromosomes in females cosegregate as a single linkage unit) , and a translocation of the two major autosomes (which, in heterozygotes, causes the two autosomal chromosomes to cosegregate as a single linkage unit, among the living offspring)
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. Males from a clonal amplification line are next crossed to one of two types of females: wild-type females from the LHM base population or females from an attached-X replica of the LHM base population that is continuously backcrossed to the LHM population (Fig.
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From page 35... ...
. Individuals within a hemiclone share half of their genetic variation in common, so that two times the additive genetic variation among hemiclones divided by the total phenotypic variation approximates the heritability of the trait in the base population.
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. Next, the vials of eggs are put through the same culturing protocol as the LHM base population, and then the numbers of offspring produced by the hemiclonal individuals are measured.
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Finally, the lifetime fecundity of the females was compared between the two treatments by measuring egg production during the last 18 h of their 2-week generation cycle (which is equivalent to egg production in the oviposition vials during the normal propagation of the LHM base population)
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From page 38... ...
However, 17% of the total genetic variation among hemiclones for lifetime fecundity was due to variation in female resistance to male-induced harm, indicating that this trait contributed substantially to total genetic fitness variation among females. Because males harm females through their seminal fluid (Chapman et al., 1995)
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From page 39... ...
corroborated a negative net selection gradient on remating rate. To test for inter-locus antagonistic coevolution between the sexes, the same 35 hemiclones were expressed in males and assayed for remating
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From page 40... ...
The logic underlying this second assay was to determine whether there was additive genetic variation for remating rate in males, whether this variation was at least partially nonoverlapping with that controlling remating in females, and whether the net-selection gradient on remating rate in males was positive. The protocol for measuring remating rate in males followed that of the male-exposed treatment of the female resistance assay except that the hemiclones were expressed as males and the females expressed random genotypes drawn from the base population (LHM-bw)
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From page 41... ...
there is unique genetic variation for remating rate in males and females that is selected and evolving in opposite directions in the two sexes. These data provide support for the hypothesis that perpetual inter-locus, intersexual arms races contribute to rapid genetic divergence among allopatric populations, and owing to the phenotypes that coevolve (reproductive behavior, physiology, and anatomy)
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From page 42... ...
, we used a laboratory island population to assess the evolutionary principles that underlie inter-locus antagonistic coevolution between the sexes. Our finding, that after hundreds of generations of coevolution we can detect an ongoing arms race between the sexes, supports the conclusion that perpetual arms races occur in nature and contribute substantially to the genetic divergence that leads to reproductive isolation and speciation.
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From page 43... ...
In this article, we show that experimental evolution, and more specifically hemiclonal analysis, provides support for the hypothesis that inter-locus antagonistic coevolution promotes rapid genetic divergence among allopatric populations. ACKNOWLEDGMENT This work was supported by National Science Foundation Grants DEB99-96164, DEB-0128780, and DEB-0410112 (to W.R.R.)
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(1999) Experimental removal of sexual selection reverses inter sexual antagonistic coevolution and removes a reproductive load.
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From page 45... ...
(2004) Female resistance to male harm evolves in response to manipulation of sexual conflict.
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