|(revised 30 October 2000)|
Animal Cognition and Culture
Genetic Wizardry Among the Ciliates
Transposons the Engine of Chromosomal Mutations?
Behaviourally Driven Gene Expression Reveals Song Nuclei In Hummingbird Brain
In some species, females choose mates possessing ornaments that predict
offspring survival. However, sexual selection by female preference for
male genetic quality remains controversial because conventional genetic
mechanisms maintain insufficient variation in male quality to account for
costly preference and ornament evolution. Here the authors show that females
prefer ornaments that indicate genetic quality generated by transmission
conflict between the sex chromosomes. By comparing sex-ratio distributions
in stalk-eyed fly (Cyrtodiopsis) progeny they found that female-biased
sex ratios occur in species exhibiting eye-stalk sexual dimorphism, and
female preferences for long eye span. Female-biased sex ratios result from
meiotic drive, the preferential transmission of a 'selfish' X-chromosome.
Artificial selection for 22 generations on male eye-stalk length in sexually
dimorphic C. dalmanni produced longer eye-stalks and male-biased progeny
sex ratios in replicate lines. Because male-biased progeny sex ratios occur
when a drive-resistant Y chromosome pairs with a driving X chromosome,
long eye span is genetically linked to meiotic drive suppression. Male
eye span therefore signals genetic quality by influencing the reproductive
value of offspring.
A. Heddi et al.
Four intracellular genomes direct weevil biology:
Nuclear, mitochondrial, principal endosymbiont, and Wolbachia
Proc. Natl. Acad. Sci. US 96 (8 Jun 99): 6814.
Cell physiology in certain cells of the weevil Sitophilus oryzae is coordinated by four integrated genomes: the nuclear genome, the mitochondrial genome, the genome of a principal endosymbiont, and the genome of a secondary endosymbiont.
The principal endosymbiont, first recognized in 1927, is a gamma-proteobacterium, present at densities of approximately 2000 bacteria per cell in so-called "bacteriocytes" or specialized insect fat cells containing endosymbiotic bacteria. The secondary (fourth) endosymbiont, discovered by the authors in three species of weevil, is an alpha-proteobacterium (Wolbachia) disseminated throughout body cells and in particularly high density in reproductive cells.
This principal endosymbiont is apparently fully integrated into the physiology of the host. It induces the specific differentiation of bacteriocytes and increases mitochondrial oxidative phosphorylation by supplying pantothenic acid, riboflavin, and biotin. By increasing mitochondrial enzymatic activity, it appears to enhance the flight ability of adult insects.
In contrast, the secondary endosymbiont has no discernible effect the physiology of the weevil, although certain genetic effects of the organism are evident under special experimental circumstances: they suggest an involvement in the speciation of the weevil. The secondary weevil endosymbiont is a virus -- a rickettsia-like organism widespread in arthropods. It is known to alter host reproduction and to be possibly virulent and lethal in the fruit fly Drosophila. In the weevil, however, the presence of this secondary endosymbiont is apparently innocuous.
The authors suggest the coexistence of two distinct types of intracellular proteobacteria at different levels of endosymbiont integration in insects illustrates the genetic complexity of animal tissue. The authors further suggest an inferred evolutionary timing: first nucleocytoplasm, then mitochondria, then the principal endosymbiont, and finally the secondary endosymbiont. Symbiogenesis, the genetic integration of long-term associated members of different species, appears in the weevil to involve a mechanism for speciation (in the case of the secondary endosymbiont) and a mechanism for the acquisition of new genes for improved environmental adaptation (in the case of the principal endosymbiont).
Contact: Abdelaziz Heddi <email@example.com>
Adaptive evolution that requires multiple spontaneous mutations.
I. Mutations involving an insertion sequence.
Genetics 120. 4 (Dec 1988): 887-97.
Escherichia coli K12 strain chi 342LD requires two mutations in the
bgl (beta-glucosidase) operon, bglR0----bglR+ and excision of IS103
from within bglF, in order to utilize salicin. In growing cells the two
mutations occur at rates of 4 x 10(-8) per cell division and less than
2 x 10(-12) per cell division, respectively. In 2-3-week-old colonies
on MacConkey salicin plates the double mutants occur at frequencies of
10(-8) per cell, yet the rate of an unselected mutation, resistance to
valine, is unaffected. The two mutations occur sequentially. Colonies that
are 8-12 days old contain from 1% to about 10% IS103 excision mutants,
from which the Sal+ secondary bglR0----bglR+ mutants arise. It is shown
that the excision mutants are not advantageous within colonies; thus,
they must result from a burst of independent excisions late in the
life of the colony. Excision of IS103 occurs only on medium containing
salicin, despite the fact that the excision itself confers no detectable
selective advantage and serves only to create the potential for a
secondary selectively advantageous mutation.