These aquaculture- and conservation-oriented commentaries are not abstracts written by the original authors.  They reflect the opinions of someone else -- usually Roger Doyle.  Direct quotations from the papers or abstracts are marked with inverted commas.

96.  Shrimp: the host vs. pathogen evolutionary arms race
Evolution of parasite virulence against qualitative or quantitative host resistance. 2000. Gandon, S., and Y. Michalakis. Proceedings of the Royal Society (UK) Series B 267:985-990.
        The authors distinguish between qualitative or absolute host resistance to a pathogen, in which there is no infection whatsoever, and quantitative or partial host resistance in which the spread of a pathogen within the host occurs but at a reduced rate. They show that "a qualitative form of resistance reduces parasite virulence, while a quantitative form of resistance generally selects for higher virulence".  This is because after successfully  infecting the host, the  variable pathogen genotypes will compete to overcome the partial resistance, and pathogen evolution by natural selection will take place.
        The paper is one of the few that studies the linked coevolution of a host and its pathogen. Most studies are concerned with the evolution of only one of the pair while the other is assumed to remain unchanged.  In shrimp aquaculture, this is not a reasonable assumption.
        The analysis is consistent with the work reported in July list #84, where a somewhat different distinction is made between host resistance (both quantitative and qualitative) and host tolerance. In the case of increasing host tolerance the virulence of a pathogen does not evolve by natural selection. This is because the relative fitness of  pathogen genotypes will not be influenced by improved tolerance.
        If we consider these two papers together, it would seem that host tolerance, for a pathogen which is always present in the population, is evolutionarily stable, and may be a desirable target for selective breeding. Absolute, qualitative resistance is also a desirable trait to select if there are any such genes available in the host population.  However, selecting for a quantitative increase in resistance appears to invite a coevolutionary arms race with the pathogen. Or in the case of an artificially applied quantitative resistance -- for instance an antibiotic dip that is less than 100% effective -- we should have a coevolutionary arms race between the pathogen and the pharmaceutical industry.
sgandon@snv.jussieu.fr  

95.  The problem for fish conservation is entropy, not extirpation
Homogenization of fish faunas across the United States. 2000. Rahel, F.J. Science 288:854-856.
        European settlers introduced fish into North America for sport and for food. "Fish faunas across the continental United States have become more similar through time. ... On average, pairs of states have 15.4 more species in common now than before European settlement of North America. The 89 pairs of states that formerly had no species in common now share an average of 25.2 species. Introductions have played a larger role than extirpations [of local endemic species] in homogenizing fish faunas." frahel@uwyo.edu 

94. Does ex situ conservation work? Loss of fish genes
Effective population size and maintenance of genetic diversity in captive-bred populations of a Lake Victoria cichlid. 2000. Fiumera , A.C., P.G. Parker, and P.A. Fuerst. Conservation Biology 14:886-892.
        The endangered cichlid species had been maintained in captive subpopulations for five generations as part of a Lake Victoria cichlid species survival plan (the major threats being over fishing and introduced Nile perch). A study of four microsatellite loci (24 alleles) showed that the effective population size was only 1% to 10% of the observed census size in the gene bank.  Not only that, around 20% of the alleles were lost in the first 4 generations. Expected diversity declined by 6% - 12% per generation. The subpopulations are being maintained in different places, in facilities that include one to several tanks of 10 - 30 adults.
       None of this sounds very good, either for genetic conservation by ex situ methods (i.e. where fish are maintained in "zoos" outside their natural habitat) or for tilapia aquaculture (to the extent that the conclusions can be generalized). The authors make several recommendations for reducing genetic loss including "periodically removing dominant males, which will encourage reproduction by additional males". The authors did not attempt to calculate the rate of accumulation of inbreeding in the populations. Readers should  think about the way inbreeding in small populations amplifies male-male competition and reduces genetic variance (July #82, June #60, May #48). fuerst.1@osu.edu  

93. All-male tilapia and opportunities for Frankensex
Sexually dimorphic expression of two types of DM (Doublesex/Mab-3)-domain genes in a teleost fish, the tilapia. 2000. Guan, G. , T. Kobayashi, and Y. Nagahama. Biochemical and Biophysical Research Communications 272:662-666.
        The authors have identified two sex-determining genes in tilapia that appear to be different versions of the "doublesex" gene first identified in Drosophila. One version is already known to be found in vertebrates (including zebra fish) and is transcribed mainly in the testis. The authors found that tilapia also has a very different variant of this same gene which is expressed in the ovary and is structurally somewhat like the female version of the Drosophila doublesex gene. The expression of the two tilapia genes is mutually exclusive in any particular fish (which must become either male or female during development), and so is presumably under the control of additional genes and environmental factors.
        This important discovery of two doublesex genes may ultimately, perhaps, help explain the spooky sex ratios that often result when tilapia species and even populations are crossed. It might also become of practical interest to aquaculture if some of the truly bizarre transgenic experiments in Drosophila could ever be replicated in fish for the purpose of controlling sexual differentiation. The authors of this paper make it clear that there are likely to be many targets for genetic intervention in the teleost sex-determining system.
        A note on Drosophila.  Drosophila has only one doublesex gene, not two like tilapia, and produces male and female versions by RNA splicing after transcription. Sexual differentiation in Drosophila is controlled by a short cascade of regulatory genes, the expression pattern of which determines all aspects of maleness and femaleness, including complex behaviors displayed by males and females. Doublesex is found near the end of the cascade. Other genes in this pathway have been given names like "transformer" and "dissatisfaction".
        Mutational and transgenic manipulation of certain genes in the cascade have produced flies that are genetic (XX) females but develop and behave as males. This genetic sex-reversal can be blocked by transforming another gene in the cascade. But if the blocking gene is coupled with a suitable promoter the blocking action can be stopped by applying a heat shock during development. Then, although the transformed female flies develop as males and display vigorous male courtship, they are still producing female pheromones and are themselves attractive to males. Their chronically amorous condition (as males) may be caused by self-stimulation from their own female pheromones. Whew! At this point we must draw a veil across their behaviour but note again that the analogous tilapia doublesex system could provide transgenic opportunities useful in aquaculture. nagahama@nibb.ac.jp 

92. Does ex situ genetic conservation work? Loss of plant genes
Genetic diversity of barley landrace accessions (Hordeum vulgare ssp. vulgare) conserved for different lengths of time in ex situ gene banks. 2000. Parzies, H.K., W. Spoor, and R.A. Ennos. Heredity 84:476-486. There are large numbers of samples of seeds of crop plants stored in gene banks throughout the world. These seeds must be planted on a regular basis and replaced by offspring seed. In the course of this rejuvenation of the collection there is an opportunity for gene frequency changes and erosion of genetic diversity due to "bottlenecking" and genetic drift.
        "In order to determine whether these undesirable effects occur, genetic diversity levels were assessed for morphological and isozyme markers within gene bank accessions of two barley landraces from Syria that had been stored for 10, 40 and 72 years. These were compared with genetic diversity levels for the same markers in barley landraces collected recently at locations in Syria where they are still under cultivation. Average gene diversity, alleles per locus and percentage polymorphic loci all showed very significant declines with length of time in storage, and genetic differentiation among accessions increased over time.... [It] was estimated that the effective population size Ne of rejuvenation populations over their period in storage was only 4.7."
        The authors also note that the "consistently high diversity of all recently collected barley landraces found in this study is a clear justification for the use of in situ conservation approaches. ("In situ" conservation means conserving organisms in their natural location and habitat, like a national park.)  rennos@ed.ac.uk  

91. How fish evolve genetic adaptation to thermal stress
Adaptive variation in lactate dehydrogenase-B gene expression: Role of a stress-responsive regulatory element. 2000. Schulte, P.M., H. C. Glémet, A.A. Fiebig, and D.A. Powers. Proceedings of the National Academy of Sciences (USA) 97:6597-6602.
        Northern and southern populations of the fish Fundulus heteroclitus have different levels of expression of the lactate dehydrogenase-B gene (Ldh-B). The northern, e.g. Newfoundland, fish are superior at lower temperatures while the southern, e.g. Florida, fish are superior at higher temperatures. The differences are known to be genetic. This exceptionally interesting study is one of the first to elucidate how adaptive gene regulation can evolve. The experimental details are too complicated to be easily summarized here, but they include temporary transgenesis of the regulatory sequences into the livers of experimental fish; deletion studies to identify the approximate location within the regulatory sequence where the adaptive changes in the transcript occurred; stress tests of live fish to see which alleles (northern or southern) drive the transcription of the gene.
        The authors found that a difference of only one base pair in the regulatory sequence accounts for the adaptive difference between the northern and southern populations.  Marker mappers and QTL selectors should take heart, but also note the experiment reported in June #65 where selection for temperature stress tolerance did not work. pschulte@uwaterloo.ca 

90. Endangered horsemeat sushi
Predicted decline of protected whales based on molecular genetic monitoring of Japanese and Korean markets. 2000. Baker, C.S., G.M. Lento, F. Cipriano, and S.R. Palumbi. Proceedings of the Royal Society (U.K) Series B 267:1191-1199.
        The authors used mtDNA data to identify the species of products sold as whale meat (which turned out to include sheep and horse as well as many species of whale, porpoise and dolphin) and sub-populations (which included a greater than expected number of samples from protected minke whales in the Sea of Japan). "The source of these products was assumed to be undocumented 'incidental takes' from fisheries' by-catch, although we cannot exclude the possibility of illegal hunting or smuggling. The demographic impact of this undocumented exploitation was evaluated using the model of population dynamics adopted by the Scientific Committee of the International Whaling Commission. For the range of exploitation consistent with the market sample, this protected stock [Sea of Japan] was predicted to decline towards extinction over the next few decades. These results confirmed the power of molecular methods in monitoring retail markets and pointed to the inadequacy of the current moratorium for ensuring the recovery of protected species." csbaker@auckland.ac.nz  

89. Selection against hatchery trout that go to sea
Microsatellite and mitochondrial DNA polymorphism reveals life-history dependent interbreeding between hatchery and wild brown trout (Salmo trutta L.). 2000. Hansen, M.M., D.E. Ruzzante, E.E. Nielsen, and K-L.D. Mensberg. Molecular Ecology 9:583-594.
        Microsatellite and mtDNA markers were examined in wild trout from stocked and unstocked sections of a Danish river. The authors concluded that genetic admixture from the hatchery trout used for stocking was much higher in the resident, non-migratory wild trout than in the migratory "sea trout" that spawn in the river but also spend time in the ocean. It appeared that more hatchery males than hatchery females are the culprits in these genetic transactions. However, this sex bias may be due to natural selection rather than sexual selection. "We suggest that stronger selection acts against stocked hatchery trout that become anadromous compared to hatchery trout that become resident. As most resident trout are males this could also explain why gene flow from hatchery to wild trout appeared to be male biased." mmh@dfu.min.dk  

88. Estimating heritabilities in aquacultural and captive populations
Estimating variance components in natural populations using inferred relationships. 2000. Thomas, S.C., J.M. Pemberton, and W.G. Hill. Heredity 84:427-436.
        Microsatellite and other marker loci have been used for several years to establish pedigrees (or at least full- and half-sib relationships) which can be used to estimate quantitative genetic parameters such as heritability of growth in natural populations. To my knowledge no one has worked out the effect of pedigree error on the accuracy of the parameter estimates. Instead, several authors have tried to estimate heritabilities and other quantitative genetic variance components from micro-satellite based relatedness calculations that do not depend on exact pedigrees. The authors carry this approach farther and evaluate the various procedures which have been developed to date.
        In this paper, two techniques have been developed which "use marker information to estimate heritabilities without reference to the exact nature of the relationships: a regression-based estimator that regresses phenotypic similarity for a pair of individuals against an estimate of their relationship and a likelihood-based estimator that maximizes the probability of the genotypic and phenotypic data given a known population structure." The could be very useful for conducting genetics experiments cheaply in commercial aquaculture environments sthomas@srv0.bio.ed.ac.uk 

87. High genetic diversity in introduced oysters
Genetic variation of wild and hatchery populations of the Pacific oyster, Crassostrea gigas (Thunberg), in Australia. 2000. English, L.J., G.B. Maguire, and R.D. Ward. Aquaculture 187:283-298.
        "Pacific oysters were introduced to Tasmania about 50 years ago from Japan; fears had been expressed that they would have lost genetic variation during their subsequent naturalisation. Using 17 allozyme loci [the authors determined that]. ...The introduced oysters appear to have retained most of the genetic variation present in the Japanese populations." The overall level of genetic variation is high but the authors acknowledge that microsatellite loci, which typically have more rare alleles, might have been more powerful for detecting genetic erosion. bob.ward@marine.csiro.au  

86. Improving the theoretical basis of conservation genetic predictions
Derivation of the relationship between neutral mutation and fixation solely from the definition of selective neutrality. 2000. Tomizawa, J-I. Proceedings of the National Academy of Sciences (USA) 97:7372-7375.
        Anyone who has taken any population genetics at all -- including the pop section of a general genetic course -- believes that the relative probabilities of fixation of neutral alleles are equal to their current relative frequencies. This commonsense prediction is actually due to M. Kimura, who used a mathematical argument that includes some assumptions and which has not been experimentally verified. Now this elegant note by Tomizawa should allow countless former genetics students who have worried about this soft spot in their educations for years and years to sleep more easily. The paper contains no formal mathematics.
        "[Tomizawa's hypothetico-deductive] approach shows that the equality of the alleles during the fixation process is equivalent to the equality of probability of their ultimate fixation in a steady state. Both are manifestations of the definition of selective neutrality. Then, solely from this dual nature of the definition, the equality between u and p is derived directly."  ("u" is the probability of fixation and "p" is the current allele frequency.) You only thought you knew this already.  The relevance to aquaculture and genetic conservation is the additional theoretical support which is given to predictions and models. tomizawa@lab.nig.ac.jp