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Mitochondrial recombination: naturally
MONITOR Mitochondrial recombination: naturally mtDNA recombination in a natural population Saville, B.J., Kohli, Y. and Anderson, J.B. Proc. Natl. Acad. Sci. U. S. A. 95, 1331–1335 Population geneticists rely heavily on the assumption that mitochondria are uniparentally inherited and that the different lineages do not recombine. This is an important precondition for tracing population genealogies and phylogenies of closely related species. However, at least in fungi, it has been known for some time that different mitochondrial lineages can, in fact, recombine if they are brought together in a single cell. Saville et al. have assessed whether mitochondrial recombination might also play a role in natural populations using the basidiomycete
fungus Armillaria gallica. A. gallica arises from a unique mating event and then grows vegetatively to form an individual that can cover many hectares and that can be hundreds of years old. Sequence analysis of a large part of the mitochondrial genome from many such individuals was employed to trace many distinct polymorphic positions for each haplotype. When these sequences were used to build a parsimony network of haplotypes, no resolvable tree could be recovered. This is extremely unusual for mitochondrial sequences, where a new
Waspish sexuality Evidence for a genomic imprinting sex determination mechanism in Nasonia vitripennis (Hymenoptera; Chalcidoidea) Dobson, S.L. and Tanouye, M.A. Genetics 149, 233–242 A universal property of the several hundred thousand species of Hymenoptera (ants, bees and wasps) is haplodiploidy: diploid females and haploid males. In some species, such as the wasp Bracon hebetor, the underlying sex determination mechanism involves female heterozygosity at one or several sex-determining loci. Inbreeding can give rise to diploid males if they become homozygous at these loci. Surprisingly, a different mechanism must operate in another
wasp, the well-studied Nasonia vitripennis, because inbreeding in this species never gives rise to diploid males. Recent research has clarified the situation in Nasonia, by making use of a selfish supernumerary chromosome called PSR (paternal sex ratio chromosome). Males carrying PSR can fertilize eggs normally, leading to diploid zygotes, but then all paternal chromosomes except PSR become condensed and are lost, resulting in more haploid
Chimeric eukaryotes Genomic evidence for two functionally distinct gene classes Rivera, M.C. et al. Proc. Natl. Acad. Sci. U. S. A. 95, 6239–6244 The era of genomic sequencing has progressed sufficiently in recent years to the point that we can now address exciting and fundamental questions about the evolution of genomes and functional gene classes in ways never before possible. Rivera et al. compared all the genes from a eukaryote, a cyanobacterium, a proteobacterium and a methanogen using a simple distance metric and, thereby, produced a representation of the evolution of the four genomes. When this representation was examined for relationships among genes of similar functional types, a telling comparison defined two functional superclasses of genes designated informational (including genes involved
in transcription, translation, replication and tRNA synthetases) and operational (including genes involved in biosynthesis, energy metabolism, and so on). Trees were then constructed from alignments of the prokaryotic and eukaryotic orthologs to examine the origin of eukaryotic nuclear genes. Coincident with many lines of thought on the subject, this analysis produced striking evidence for a chimeric origin of eukaryotes. The surprising finding, however, was that the different functional classes of genes have apparently arisen from different types of prokaryotes. Eukaryotic operational genes are derived primarily from proteobacterial genes, with significant contributions TIG AUGUST 1998 VOL. 14 NO. 8
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0168-9525/98/$19.00
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haplotype should only arise by a single mutation from an existing haplotype. Because Saville et al. can also show that homoplamy is unlikely to play a role, one is forced to accept that recombination events must have occurred. Most interestingly, analysis of linkage of polymorphic sites shows that almost all are in linkage equilibrium, suggesting that recombination is not rare, although an exact rate cannot yet be given. Recombination would of course not be a problem in species with a strict maternal inheritance of mitochondria. However, low levels of paternal inheritance can only seldomly be excluded and could result in recombined mitochondrial haplotypes in populations. It should therefore be worthwhile to consider this possibility when analysing mitochondrial polymorphism data in the future. ✍
PSR males. So PSR males sire all-son broods when crossed with diploids. The significant new experiment involves crossing such PSR males with triploid females. This should give rise to triploid embryos, but PSR again leads to the loss of the paternal set of chromosomes. The embryos therefore mature as diploids, and these develop into males, not females. The observations are inconsistent with a variety of previous explanations for sex in N. vitripennis, but support a model in which sex is determined by some kind of imprint on the paternal set of chromosomes, which dictates female development. If the paternal set is absent, or destroyed by PSR, then the imprint is absent and male development occurs. ✍
from cyanobacteria and methanogens, whereas eukaryotic informational genes are derived almost exclusively from methanogen genes. The analysis further indicates that the operational gene lineage has diverged more recently than the informational lineage, possibly indicating that horizontal gene transfer occurs relatively more easily, or frequently, for operational genes. The reasons and mechanisms underlying this differential evolution will be fascinating, indeed. ✍
Monitor contributors this month Andrew Stoker, Diethard Tautz, Jonathan Hodgkin and Stephen Mathias
fungus Armillaria gallica. A. gallica arises from a unique mating event and then grows vegetatively to form an individual that can cover many hectares and that can be hundreds of years old. Sequence analysis of a large part of the mitochondrial genome from many such individuals was employed to trace many distinct polymorphic positions for each haplotype. When these sequences were used to build a parsimony network of haplotypes, no resolvable tree could be recovered. This is extremely unusual for mitochondrial sequences, where a new
Waspish sexuality Evidence for a genomic imprinting sex determination mechanism in Nasonia vitripennis (Hymenoptera; Chalcidoidea) Dobson, S.L. and Tanouye, M.A. Genetics 149, 233–242 A universal property of the several hundred thousand species of Hymenoptera (ants, bees and wasps) is haplodiploidy: diploid females and haploid males. In some species, such as the wasp Bracon hebetor, the underlying sex determination mechanism involves female heterozygosity at one or several sex-determining loci. Inbreeding can give rise to diploid males if they become homozygous at these loci. Surprisingly, a different mechanism must operate in another
wasp, the well-studied Nasonia vitripennis, because inbreeding in this species never gives rise to diploid males. Recent research has clarified the situation in Nasonia, by making use of a selfish supernumerary chromosome called PSR (paternal sex ratio chromosome). Males carrying PSR can fertilize eggs normally, leading to diploid zygotes, but then all paternal chromosomes except PSR become condensed and are lost, resulting in more haploid
Chimeric eukaryotes Genomic evidence for two functionally distinct gene classes Rivera, M.C. et al. Proc. Natl. Acad. Sci. U. S. A. 95, 6239–6244 The era of genomic sequencing has progressed sufficiently in recent years to the point that we can now address exciting and fundamental questions about the evolution of genomes and functional gene classes in ways never before possible. Rivera et al. compared all the genes from a eukaryote, a cyanobacterium, a proteobacterium and a methanogen using a simple distance metric and, thereby, produced a representation of the evolution of the four genomes. When this representation was examined for relationships among genes of similar functional types, a telling comparison defined two functional superclasses of genes designated informational (including genes involved
in transcription, translation, replication and tRNA synthetases) and operational (including genes involved in biosynthesis, energy metabolism, and so on). Trees were then constructed from alignments of the prokaryotic and eukaryotic orthologs to examine the origin of eukaryotic nuclear genes. Coincident with many lines of thought on the subject, this analysis produced striking evidence for a chimeric origin of eukaryotes. The surprising finding, however, was that the different functional classes of genes have apparently arisen from different types of prokaryotes. Eukaryotic operational genes are derived primarily from proteobacterial genes, with significant contributions TIG AUGUST 1998 VOL. 14 NO. 8
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0168-9525/98/$19.00
306
haplotype should only arise by a single mutation from an existing haplotype. Because Saville et al. can also show that homoplamy is unlikely to play a role, one is forced to accept that recombination events must have occurred. Most interestingly, analysis of linkage of polymorphic sites shows that almost all are in linkage equilibrium, suggesting that recombination is not rare, although an exact rate cannot yet be given. Recombination would of course not be a problem in species with a strict maternal inheritance of mitochondria. However, low levels of paternal inheritance can only seldomly be excluded and could result in recombined mitochondrial haplotypes in populations. It should therefore be worthwhile to consider this possibility when analysing mitochondrial polymorphism data in the future. ✍
PSR males. So PSR males sire all-son broods when crossed with diploids. The significant new experiment involves crossing such PSR males with triploid females. This should give rise to triploid embryos, but PSR again leads to the loss of the paternal set of chromosomes. The embryos therefore mature as diploids, and these develop into males, not females. The observations are inconsistent with a variety of previous explanations for sex in N. vitripennis, but support a model in which sex is determined by some kind of imprint on the paternal set of chromosomes, which dictates female development. If the paternal set is absent, or destroyed by PSR, then the imprint is absent and male development occurs. ✍
from cyanobacteria and methanogens, whereas eukaryotic informational genes are derived almost exclusively from methanogen genes. The analysis further indicates that the operational gene lineage has diverged more recently than the informational lineage, possibly indicating that horizontal gene transfer occurs relatively more easily, or frequently, for operational genes. The reasons and mechanisms underlying this differential evolution will be fascinating, indeed. ✍
Monitor contributors this month Andrew Stoker, Diethard Tautz, Jonathan Hodgkin and Stephen Mathias