Many investigators have attempted to confirm the prediction that increased levels of heterozygosity entail greater developmental stability, manifesting itself through decreased phenotypical ...variation. The evidence presented so far is equivocal. The predicted relationship has been found in some morphological studies, but not in others. I propose that the variability of a character should be seen as different from the character itself. For most morphological characters, natural selection promotes strong canalization of development but, to facilitate responses to environmental changes, the organism needs to retain malleability of physiological and behavioral traits. These different types of selection should lead to distinct genetic architectures for these phenotypes. I report on the results of a diallel cross between four inbred mouse strains. Qualitatively different genetic architectures were in fact revealed for variation in behaviors in the open-field. In a second study, variances of inbred and hybrid populations for hippocampal morphometry were studied. Again, hybrids were not always less variable than inbreds and sometimes even more variable. It follows that there exists no one-to-one relation between heterozygosity and developmental stability.
Amplified fragment length polymorphism DNA markers have been used to assist plant breeders in the choice of maize parents for commercial hybrid production. However, maize yield in Tanzania is ...significantly reduced by gray leaf spot (GLS) disease, which is now regarded as the most yield limiting disease of maize worldwide. Thus combining GLS resistance genes and high yielding traits in hybrids is an important breeding strategy. The main aim of this study was to assess the genetic diversity of Tanzanian germplasm and to predict the potential of these inbreds in producing high yielding GLS resistant hybrids. Furthermore, the potential of such data to predict the best line combiners was investigated. This research used AFLP-DNA fingerprinting data from 21 moderately and highly GLS resistant maize inbreds. Results revealed that the genetic distance (GD) of some intergroup crosses were as high as 0.5. Theoretically, these intergroup hybrids with high GD could potentially produce high yielding GLS resistant hybrids. However, such hybrids would require field-testing in order to confirm these observations. Finally the results revealed high f values consistent with other reports in maize. In summary these results also corroborate the usefulness of AFLP in genetic diversity studies of germplasm, prediction of best line combiners and high heterosis level for commercial maize hybrid production.
Long-sleep (LS) and short-sleep (SS) mice, initially selected for differential sensitivity to ethanol, also exhibit differential sensitivity to propofol. By interbreeding LS and SS mice to obtain ...progeny whose chromosomes are a patchwork of the LS and SS chromosomes, the authors determined whether differential propofol sensitivity cosegregates with any particular chromosomal region(s). Such cosegregation is the essence of genetic linkage mapping and a first step toward isolating a gene that can modulate propofol sensitivity in mammals. A gene underlying a quantitative trait such as anesthetic sensitivity is commonly called a quantitative trait locus (QTL).
The propofol dose was 20 mg/kg injected retroorbitally. Sensitivity was measured as the duration of the loss of righting reflex (LORR). The LORR and propofol brain levels at awakening were determined for 24 LSXSS recombinant-inbred (RI) strains, derived by intercrossing LS and SS for two generations followed by >20 generations of inbreeding. A genetic linkage between LORR and an albino mutation on chromosome 7 was investigated further using 164 second-generation progeny (F2s) from intercrossing inbred LS and inbred SS mice, similar to the LSXSS RIs except F2s are not inbred. The linkage between propofol sensitivity and the albino locus also was investigated using additional genetic markers on chromosome 7. Statistical significance was assessed by interval mapping using a regression method for RIs and Mapmaker/QTL (Whitehead Institute, Cambridge, MA) for F2s.
Genetic mapping in the LSXSS RIs revealed a QTL tightly linked to the Tyr (albino) locus that accounts for nearly all of the genetic difference in propofol sensitivity between LS and SS mice. Analysis of propofol brain levels at awakening indicated that this QTL results from differential neurosensitivity. Mapping in F2s confirmed the genetic linkage to Tyr. Mice (ISS c/c x C57BL/6 c2j/C) that differed only by an albino mutation at Tyr were not differentially sensitive to propofol.
A single QTL, called Lorp1, underlies most of the genetic difference in propofol neurosensitivity between LS and SS mice. Although this QTL is tightly linked to Tyr, propofol sensitivity is not modulated by albinism. For mapping this QTL, the LSXSS RIs proved to be an especially powerful resource, localizing the candidate-gene region to a 99% confidence interval of only 2.5 centimorgans.
Johnson and Wood constructed recombinant inbred strains of Caenorhabditis elegans with life spans ranging from 10 to 31 days. Using these strains, we have demonstrated previously that hyperoxia and ...methyl viologen inhibited development at rates inversely correlated with life span. The growth rates of the short-lived recombinant inbred strains were more profoundly inhibited by oxidative stress than were those of the long-lived strains. Here we report a positive correlation between life span and catalase levels in these same strains. Specifically, when compared to short-lived strains at 10 days after fertilization, the long-lived strains possessed higher levels of total enzymatic catalase. Northern blots indicated a similar relationship between life span and clt-1mRNA (the cytosolic catalase). This suggests that at least some of the polygenes that influence life span are also responsible for regulating gene expression of catalase, an important defense component against oxidative stress.
Downy mildews cause considerable damage to maize (Zea mays L.) worldwide, particularly in the tropical Asia. We have evaluated a set of 42 tropical/sub-tropical maize inbred lines developed in ...different countries in Asia (India, Thailand and Philippines), and Mexico, for analysing the genetic variability for resistance to sorghum downy mildew Peronosclerospora sorghi; SDM and Rajasthan downy mildew P. heteropogoni; RDM. Experiments were carried out in replicated trials under artificial infection in field conditions against SDM and RDM at Mandya in Karnataka, India, and Udaipur in Rajasthan, India, respectively, during 1999 and 2000. The study resulted in identification of five inbred lines offering consistent and strong resistance to both SDM and RDM, while several inbred lines revealed resistance only to RDM. It was also revealed that the SDM-resistant inbreds are invariably resistant to RDM, while the RDM-resistant inbreds might show differential responses to the SDM. The maize inbred lines identified in this study with broad-spectrum resistance to downy mildews could be potentially useful for basic and applied research work on downy mildews in tropical Asia.PUBLICATION ABSTRACT
Corn (Zea mays L.) inbreds and hybrids may respond differently to specific herbicides. Screening corn inbreds and hybrids for tolerance to herbicides is essential as this will enable producers to ...reduce the risk of crop injury by applying appropriate herbicides. A study was conducted to determine the tolerance of corn to herbicides and to compare results in the greenhouse and in the field. Both inbreds and hybrids demonstrated significant differences in their response to herbicides. Hybrids were generally more tolerant than inbreds. Of the herbicides, metazachlor was the most injurious to corn genotypes. Differential recovery from herbicide injury was observed in the field, and some genotypes recovered fully during the growing season while others did not. Some genotypes showed significant reductions in shoot dry mass while their root dry mass was not reduced; the opposite effect was also noted for certain genotype/herbicide combinations. Percentage reduction in grain yield in the field was positively and significantly correlated with shoot dry mass reduction of seedlings in the greenhouse and in the field, as well as with visual injury rating done in the greenhouse and field. In view of the strong correlation between key parameters of herbicide sensitivity, it is suggested that screening procedures could be expedited by basing decisions mainly on results from experiments done under controlled conditions and, hence, doing only selective testing in the field in order to verify extrapolation of findings.
Downy mildews are among the most destructive diseases affecting maize production and productivity in various tropical Asian Countries, including India. In view of the scarce information on genetic ...variability for resistance to sorghum downy mildew (Peronosclerospora sorghi; SDM) and Rajasthan downy mildew (P. heteropogoni; RDM) in India, experiments were undertaken under rigorous artificial infection conditions during 1999 and 2000, at Mandya, in Karnataka, and Udaipur, in Rajasthan, for characterization of the responses of 49 Indian maize inbreds to the SDM and RDM pathogens, respectively. The study resulted in identification of an inbred line, NAI116, that offers high level of resistance to both the downy mildews, besides ten inbred lines that offer resistance to RDM. However, majority of the Indian maize inbreds were vulnerable to both SDM and RDM. Two inbred lines, KUI1411 and KUI1414A, which were reported to be resistant to P. zeae in Thailand, displayed high susceptibility to SDM and resistance to RDM. Identification of a stable source of resistance to major downy mildew diseases in India offers opportunities for undertaking detailed genetic and molecular analyses of downy mildew resistance
Le peronospore sono fra le malattie più dannose per la produzione maidicola in vari Paesi asiatici tropicali, tra questi l´India. Considerate le informazioni molto limitate sulla variabilità genetica relativa alla resistenza alla peronospora del sorgo (Peronosclerospora sorghi; SDM) e alla peronospora del Rajasthan (P. heteropogoni; RDM) in India, sono stati intrapresi esperimenti in condizioni rigorose di infezione artificiale, nel corso del 1999 e del 2000, a Mandya, Karnataka, e Udaipur, Rajasthan, per la caratterizzazione delle risposte di 44 linee pure, rispettivamente, agli agenti patogeni di SDM ed RDM. Lo studio ha portato all´identificazione di una linea pura, NAI116, che presenta un livello elevato di resistenza ad ambedue le peronospore, inoltre di 10 linee che presentano resistenza a RDM. Tuttavia, la maggior parte delle linee pure indiane di mais risultava suscettibile sia a SDM, sia a RDM. Due linee, KUI411 e KUI414A, riferite come resistenti a P. zeae in Tailandia, hanno manifestato elevata suscettibilità a SDM e resistenza a RDM. L´identificazione di una fonte stabile di resistenza alle principali forme di peronospora in India consente di intraprendere analisi genetiche e molecolari dettagliate della resistenza alla peronospora.
Theoretical comparisons for quantitativetrait loci (QTL) mapping properties wereconducted among simulated recombinantinbred (RI) populations developed bysingle-hill (SH), complete bulk, and ...singleseed descent (SSD) procedures by MonteCarlo simulations based on variouspopulation sizes, heritabilities, and QTLeffects. Our simulations includedestimation of QTL effects, QTL positions,and statistical testing power in the RIpopulations by comparing the estimates withpreset values. The simulation resultsshowed that the single hill (SH) bulk andsingle seed descent RI populations weregenerally not significantly different withrespect to quality of estimated QTL effectsand positions. Furthermore, when each RIpopulation had 150 lines, each couldprovide desirable properties for QTLmapping. The results implied that a SH RIpopulation consisting of 75 or moreF^sub 2^-derived families with two lines perfamily (corresponding population size of150 or above) was appropriate for QTLmapping and was not significantly differentthan a SSD RI population of 150. Thus, theSH method could be used to develop largenumbers of RI lines for achieving betterresults in QTL mapping. Simulations alsoshowed that there was no significantdifference between means using SH methodswith 10 and 100 fruits per family. However, RI populations developed by thecomplete bulk method where F^sub 2^identities are lost were not suitable forQTL mapping.PUBLICATION ABSTRACT
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