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.

310.  Shrimp markers, shrimp map
         Genetic mapping of the black tiger shrimp Penaeus monodon with amplified fragment length polymorphism. 2002. Wilson, K., Y. Li, V. Whan, S. Lehnert, K. Byrne, S. Moore, S. Pongsomboon, A. Tassanakajon, G. Rosenberg, E. Ballment, Z. Fayazi, J. Swan, M. Kenway, and J. Benzie. Aquaculture 204 (3-4):297-309.
         Twenty-three AFLP primers were used to find 673 polymorphic loci which were free from significant segregation distortion. Of these marker loci, 116 which were found to be segregating in more than one of the experimental families were used to construct a low density linkage map. Some microsatellite loci were also used. Twenty distinct linkage groups were identified. There are, however, more than 40 chromosome pairs in P. monodon and the authors say that there are other large gaps in their preliminary map.
         This looks like the start of a serious mapping program, and it is. The authors invite contributions from other researchers who have markers that can be placed on the P. monodon map. The Shrimp-Map project has a website: http://www.aims.gov.au/pages/research/shrimpmap/pages/sm-00.html . Presumably details on the mapped primers and microsatellites are freely available from the site or from the authors although I haven't checked. k.wilson@aims.gov.au  or  yutao.li@li.csiro.au .

309.  Oops! Sorry! Not eureka!
         Population admixture may appear to mask, change or reverse genetic effects of genes underlying complex traits. 2002. Deng, H.-W. Genetics 159:1319-1323.
         If you find a statistical association between a molecular marker and disease susceptibility or resistance in a single generation of an aquaculture broodstock, the association will almost always be false. This is because aquaculture broodstocks usually consist of relatively few groups of siblings and/or admixed populations which have somewhat different marker gene frequencies.
         The author of this paper published a useful analysis a year ago (Mar-April 2001 #182) which showed that when populations mixtures are used to search for QTLs you get a lot of false positive associations, and you may go chasing after genes that aren't there.
         The new paper goes further in analysing "the effects and the conditions of population admixture in masking, changing, or even reversing true genetic effects of genes underlying complex traits".
         The possible solutions include controlled breeding and progeny testing etc., or doubled haploid trickery (May 2000 #59, Jan 2002 #279), or perhaps even work in silico (Aug 2001 #217). deng@creighton.edu .

308.  Aureus has two sex chromosomes!
         Identification of putative sex chromosomes in the blue tilapia, Oreochromis aureus, through synaptonemal complex and FISH analysis. 2001. Campos-Ramos, R. , S.C. Harvey, J.S. Masabanda, L.A.P. Carrasco, D.K. Griffin, B.J. McAndrew, N.R. Bromage, and D.J. Penman. Genetica 111 (1-3):143-153.
         In O. aureus the heterogametic sex is thought to be female, unlike O. niloticus, O. mossambicus and H. sapiens where it is the male, most of the time. In tilapias the heterogametic sex chromosomes cannot be distinguished by their appearance as seen through a microscope. This paper is based on a wonderful optical and electron microscopic study of that magical meiotic moment when homologous chromosomes are tightly paired and exchange of genetic material between paired DNA molecules is taking place. Pairing and exchange does not occur between non-homologous chromosome regions, and the authors noted two unpaired regions in nuclei of female O aureus.
         The two regions were on different chromosomes, one of which has not yet been identified in the karyotype. There were no unpaired regions in aureus males. One of the unpaired regions is closely related to the sex-determining region of male niloticus which, however, has only one such region. This paper appears to be the first demonstration of two pairs of probable sex chromosomes in a single species. Therefore it is hardly surprising that species hybrids involving aureus have sex ratios which are strange and unpredictable, but sometimes useful, not only in the F1 but in descendent generations as well. d.j.penman@stir.ac.uk .

307.  One immigrant per generation will not prevent inbreeding
         Migration and inbreeding: the importance of recipient population size for genetic management. 2001. Vucetich, J.A., and T.A. Waite. Conservation Genetics 2 (2):167-171.
         This paper makes the useful point that in real populations the number of immigrants needed to prevent inbreeding is actually much greater than one individual per generation, which is the theoretical requirement in idealized Fisher-Wright populations.
         In random-mating populations where reproductive variance follows a Poisson distribution one immigrant per generation will theoretically do the trick if the numerical population size is larger than about 20. However, variation in mating success, fecundity etc in real populations increases reproductive variance and causes the effective population size to be considerably less than the numerical (census) size.
         This may be well known, but the authors point out that the reproductive variance of  immigrants is highly variable as well. The result is that more than one immigrant individual is needed to prevent inbreeding and the situation becomes worse the greater the discrepancy between actual and effective population sizes. Most importantly, the required number of immigrants increases with the census size of the population, which is not the case in idealized, theoretical populations in which the census and effective sizes are equal.
         It is interesting to compare this theoretical study with a field study of the effect of a single immigrant on a bottlenecked population of warblers (Sep 2001 #235). In that real-world population, selection (purging) apparently removed the alleles responsible for inbreeding depression, but the immigrant promptly brought deleterious alleles back in again! javuceti@mtu.edu .

306.  Cost-effective breeding design for aquaculture genetics?
         Polymix breeding with parental analysis of progeny: an alternative to full-sib breeding and testing. 2001. Lambeth, C., B.-C. Lee, D. O'Malley, and N. Wheeler. Theoretical and Applied Genetics 103 (6/7):930-943.
         This forest genetics paper describes a simple breeding design for estimating breeding values that might be directly transferred to aquaculture. Just mix pollen (sperm) from a lot of males together and fertilize some females with the mixture. Then use DNA markers to sort out the pedigrees, and use the resulting relationship matrix and performance data to estimate breeding values of either the parents or the offspring. The paper has a lot of technical analyses of statistical power and efficiency, but basically, what could be easier than this? One major complication is the variance in reproductive success in aquaculture (#304, below) which might greatly increase the number of DNA analyses required to complete the genetic analysis of the polymix offspring. clem.lambeth@weyerhaeuser.com .

305.  Markers identify individual shellfish larvae
         Microsatellite genotyping of individual abalone larvae: parentage assignment in aquaculture. 2001. Selvamani, M.J.P., S.M. Degnan, and B.M. Degnan. Marine Biotechnology 3:478-485.
         PCR procedures are sensitive enough so ten microsatellite loci can be analysed on each veliger larva. Five were used for parentage analysis and three loci were generally sufficient to assign parentage in this experiment. This can be an efficient way to set up a genetics program in aquaculture. The authors found, moreover, that most fertilizations were attributable to just one of the males in the sperm mix. See Nov 2001 #259 on extreme reproductive variance in elm-oyster species, and the comment in #306, above, on the complications this might cause in "polymix" mating designs. bdenan@zoology.uq.edu.au .

304.  Variable oyster reproduction decreases effective population number
         High variance in reproductive success of the Pacific oyster (Crassostrea gigas, Thunberg) revealed by microsatellite-based parentage analysis of multifactorial crosses. 2002. Boudry, P., B. Collet, F. Cornette, V. Hervouet, and F. Bonhomme. Aquaculture 204 (3-4):283-296.
         The authors of this paper used a single, highly variable, microsatellite locus to trace the sources of variation in the reproductive success of laboratory crosses of C. gigas. "Results show large variance in parental contributions at various developmental stages, leading to a strong reduction of experiment-wide effective population sizes. Segregation distortions fluctuating with time were also observed. ... observed variance in reproductive success can be attributed to three main factors: gamete quality, sperm-egg interaction and differential viability among genotypes." The authors also found that sperm competition increased reproductive variance and decreased effective population size.
         These results are qualitatively similar to another recent paper on the same species (Nov 2001 #259). Paper #259 elegantly wrapped developmental mortality, segregation distortion, inbreeding depression and compulsory sex into a general theory of life history strategies known as Williams's elm-oyster model. Elm-oyster species produce enormous numbers of offspring which they broadcast into a hostile world. The outlook for any particular larva is not good, not least because it usually has a  genotype which is unfit everywhere.
         The practical implication for maintaining diversity in hatcheries is evident. Effective population number will be particularly hard to maintain in elm-oyster species. The danger of inbreeding depression will be particularly severe because (a) it is likely to happen and (b) when it does happen it will cause serious problems. pboudry@ifremer.fr

303.  Marker relatedness estimator agrees with a very long pedigree record
         Microsatellite diversity, pedigree relatedness and the contributions of founder lineages to thoroughbred horses. 2001. Cunningham, E.P., J.J. Dooley, R.K. Splan, and D.G. Bradley. Animal Genetics 32 (6):360-364.
         Thoroughbred horse pedigrees extend back about 300 years and offer a good opportunity to check the accuracy of marker-based relatedness estimators (Sep 2001 #227). This paper used a shared-allele estimator calculated from data on twelve microsatellite loci. Agreement between relatedness calculated from pedigree records and estimated from the microsatellites was excellent. The correlation between the recorded pedigrees and relatedness estimators is good news for aquaculture and fisheries conservation, where pedigrees are rare and expensive to maintain. Marker-based relatedness estimators may come into use for estimating variance components (Aug 2000 #88) and inbreeding (Dec 2001 #269) and for preserving founder diversity in hatcheries (Nov 2001 #261; Jan 2002 #283). epcnnghnn@tcd.ie

302.  More natural selection for MHC diversity in salmon
         Geographic heterogeneity in natural selection on an MHC locus in sockeye salmon. 2001. Miller, K.M., K.H. Kaukinen, T.D. Beacham, and R.E. Withler. Genetica 111 (1-3):237-257.
         It is becoming evident that high diversity at the important MHC loci is often, or usually, maintained by modes of selection that favour diversity per se rather than particular alleles. These selection modes include mate choice (Jul-Aug 2001 #221) and possibly overdominant selection for disease resistance (October 2001 #240). This study of 31 populations of sockeye salmon, like another recent study on Atlantic salmon (December 2001 #272), demonstrates that balancing selection which increases diversity takes place locally, within populations.
         There is however also evidence for directional selection at the MHC in several of these sockeye populations. The Atlantic salmon study found that genetic distances between populations as measured at the MHC locus correlated reasonably well with genetic distances measured at neutral microsatellite loci and also with geographic distance. The Atlantic authors concluded that divergence of populations is essentially a random process.
         The sockeye authors reach a different conclusion. "The apparent heterogeneity in selection [at MHC loci] resulted in strong genetic differentiation between geographically proximate populations with and without detectable levels of balancing selection, in stark contrast to observations at neutral loci." The paper finishes with "The distribution of MHC class II diversity throughout the Fraser drainage supports the contention that conservation of sockeye salmon must be conducted on the basis of individual lake systems." Millerk@pac.dfo-mpo.gc.ca .

301.  Selection for larval behaviour changes adult traits
         The effects of selection for larval behavior on adult life-history features in Drosophila melanogaster. 2001. Foley, P.A., and L.S. Luckinbill. Evolution 55 (12):2493-2502.
         Domestication selection on larval feeding behaviour undoubtedly takes place in hatcheries operated both for aquaculture and genetic conservation. Presumably hatchery selection favours larvae that eat more and faster, although that may not be the case in some culture systems. Anyway we know essentially nothing about what the genetic consequences of such selection might be, which is why this experiment on Drosophila is interesting as a model.
         The authors found a tradeoff between fitness traits. Lines selected for high feeding rates as larvae grew faster but had reduced life spans. Lines selected for a low larval feeding rate grew more slowly to adulthood but had lower mortalities and also enhanced expression of genes known to promote resistance to stress. This may be interesting (by analogy) in the light of possible growth-survival tradeoffs which has been reported in shrimp (e.g. Feb 2002 #290). lluckin@biology.biosci.wayne.edu

300.  Relaxing intensity of selection does indeed reduce fitness
         The relative effects of mutation accumulation versus inbreeding depression on fitness in experimental populations of the housefly. 2001. Reed, D.H., and E.H. Bryant. Zoo Biology 20 (3):145-156.
         The authors of this paper bred large (500) and small (50) replicate housefly population in a regime which eliminated selection on traits that are expressed after 21 days of age. They accomplished this by killing all the flies at 21 days. This is a useful model experiment for aquaculture or genetic conservation because, when breeding captive populations, it is hard to know which of several potential genetic problems to worry about most. Here are three major worries (which does not however exhaust the list of possible worries):
         Worry #1: deliberate or incidental selection may change the value of traits away from their natural optima (domestication selection).
         Worry #2: Reduced natural selection in the benign, domestic environment, will permit unfavorable genes to accumulate in the population by drift and/or mutation pressure even though they are not selected (relaxed selection). The result may be a catastrophic loss of fitness when the organisms are exposed again to the full force of natural selection, e.g. in fisheries stock-enhancement or in some types of aquaculture. Note that both of these effects (worry #1 and worry #2) can occur in captive populations of any size.
         Worry #3: If the captive population is small, inbreeding accumulates as the generations pass. This occurs because the small sample of gametes in each generation leads rapidly to the accumulation of deleterious homozygotes – unless they are purged by selection. See "mutational meltdown" February 2001 #171, "extinction vortex" June 2001 #210; and the unusual theoretical perspective provided by January 2001 #157.
         This model experiment with a small, captive population of flies provides a useful reality check. " [The] rate of loss [of later-life fitness components] due to relaxed selection was equivalent to the rate of loss due to inbreeding in populations with an effective size of 50 individuals. Even if captive populations are kept large to avoid inbreeding, breeding them in benign environments where the forces of natural selection are curtailed may jeopardize the capability of these populations to exist in natural environments within few generations." ebryant@fone.net

299.  Lonely, crooked, depressed, and venomous
         Low genetic diversity threatens imminent extinction for the Hungarian meadow viper (Vipera ursinii rakosiensis). 2002. Újvári, B., T. Madsen, T. Kotenko, M. Olsson, R. Shine, and H. Wittzell. Biological Conservation 105127-130 (1).
         This is a good example of what can happen to the genetics of small, isolated, remnant populations. When compared with much larger populations in Ukraine, the Hungarian snakes have low MHC variability (presumably reducing the immune response), and greater genetic uniformity. "In combination with reports of birth deformities, chromosomal abnormalities and low juvenile survival, these data strongly suggest that the Hungarian vipers are experiencing inbreeding depression.... Given the very low numbers of animals, the only feasible strategy to increase the genetic diversity and to save the Hungarian vipers from extinction is to implement a captive breeding program based on genetically screened animals." thomas.madsen@zooekol.lu.se.

298.  Successful mass selection for growth in aquaculture
         Stock improvement of silver barb (Barbodes gonionotus Bleeker) through several generations of genetic selection. 2002. Hussain, M.G., M.S. Islam, M.A. Hossain, M.I. Wahid, A.H.M. Kohinoor, M.M. Dey, and M.A. Mazid. Aquaculture 204 (3-4):469-480.
         Mass selection was performed on a composite base population which consisted of reciprocal crosses among fish originating from Thailand, Indonesia and Bangladesh. The pure Bangladesh stock was used as an unselected reference population. "A 7.5% genetic gain in growth performance was attained by the F1 crossbred group over the nonselected control group. ...The weight gain values of the third generation of the selected group showed 21.9% superiority over the nonselected control." It is nice to see a report on mass selection that actually works with a tropical fish species. Hussain e-mail fsbfri@bdonline.com .