Recurring origins of allopolyploid species in asplenium

A large proportion of plant species has originated through allopolyploidy: interspecific hybridization followed by chromosome doubling. Heterozygosity remains fixed in allopolyploids because of nonsegregation of parental chromosomes. Two allotetraploid species of the fern genus Asplenium show allozyme polymorphisms at loci that are polymorphic in their diploid progenitors, indicating that each has originated more than once and implicating continued gene flow from diploids to tetraploids.     

Periodicity in tree rings from the corn belt

Previous tree-ring studies indicated that the total area affected by drought in the western United States has rhythmically expanded and contracted over the past 300 years, with a period near the 18.6-year lunar nodal and 22-year double-sunspot cycles. Recently collected tree-ring data from the U.S. Corn Belt for the years 1680 to 1980 were examined for evidence of either of these cycles on a regional scale. Spectral analysis indicated no periodicity in the eastern part of the Corn Belt, but a significant 18.33-year period in the western part. The period length changed from 17.60 to 20.95 years between the first 150 years and the last 151. High-resolution frequency analysis showed that the structure of the 18.33-year spectral peak was complex, with contributions from several frequencies near both the lunar nodal and double-sunspot periods. A t-test of difference of means in reconstructed annual precipitation weakly corroborated a previous finding of an association between drought area and the phase of the double-sunspot cycle. Both the high-resolution frequency analysis and the t-test results indicate that the periodic component of drought near 20 years is too weak and irregular to be of use in drought forecasting for the Corn Belt.     

Margin to craton expansion of late ordovician benthic marine invertebrates

A biostratigraphic survey of 57 Late Ordovician marine shelly invertebrates from the Climacograptus manitoulinensis zone of eastern Canada supports suggestions that throughout the Early Phanerozoic benthic marine speciations occurred preferentially in marginal marine environments. The species subsequently spread onto the craton. There is no obvious positive correlation between the times of first appearance of new associations or novel communities along the continental margin and the first appearance on the craton of the species making up these communities. Taxonomic similarities between marine communities that occupied both marginal and cratonic regimes may reflect a more static local ecology than the evolutionary dynamics of a piecemeal species-by-species reassembly.     

A bacteriological investigation of subcutaneous abscesses in cats

Ten out of eleven subcutaneous abscesses in cats harboured anaerobic bacteria. Seven of the ten abscesses contained a mixture of obligate anaerobes and facultative anaerobes or microaerophilic bacteria. All of the anaerobic isolates were sensitive to the antibiotic metronidazole, but were resistant to streptomycin. Three isolates from one abscess were resistant to penicillin and ampicillin. Clinical resolution of subcutaneous abscesses was poor when procaine penicillin was used alone as treatment.     

Sources of variation and prospects for improvement of productive efficiency in the dairy cow: a review

In this review, "productive efficiency" in dairy cows is defined as the yield of milk obtained in ratio to the nutritional costs associated with maintenance, milk synthesis and loss of body condition during lactation. Improvements in efficiency could occur as a result of changes in digestion and nutrient absorption, maintenance requirement, utilization of metabolizable energy for production or nutrient partitioning. Digestibility can be greatly enhanced by appropriate dietary manipulation. Likewise, it may be possible to reduce maintenance requirements and improve the efficiency with which metabolizable energy is used for milk synthesis by manipulation of the pattern of nutrients presented to tissues. However, these factors apparently do not respond to selection for increased milk yield, and little variation is observed among cows. In contrast, individual cows differ substantially in feed intake and in the partitioning of nutrients among body tissues. Techniques associated with genetic engineering and the early prediction of genetic merit have the potential to improve productive efficiency by manipulation of these processes. However, changes in nutrient partitioning and feed intake during lactation are coordinated by a complex network of controls that accommodate the nutrient requirements of each tissue while maintaining homeostatic balance. Future improvements in productive efficiency will therefore depend on our ability to understand the manner in which these controls operate.