Fitness effects of amino acid replacements in the beta-galactosidase of Escherichia coli

Two genetic procedures were used to obtain amino acid replacements in the lacZ-encoded beta-galactosidase in Escherichia coli. Amino acid replacements could be obtained without regard to their effects on lactase activity by selecting spontaneous mutations that relieved the strong polarity of six nonsense mutations. When streaked on MacConkey- lactose indicator plates, approximately 75% of these mutants gave strong red lactose-fermenting colonies, and 25% gave white nonfermenting colonies. Mutants from 11 other nonsense codons were isolated directly using MacConkey-lactose indicator plates, on which positive color indication requires only 0.5% of the wildtype lactase activity. Among the total of 17 codons, 25 variant beta-galactosidases were identified using electrophoresis and thermal denaturation studies. The fitness effects of these variant beta-galactosidases were determined using competition experiments conducted with lactose as the sole nutrient limiting the growth rate in chemostat cultures. Three of the replacements were deleterious, one was selectively advantageous, and the selective effects of the remaining 21 were undetectable under conditions in which the smallest detectable selection coefficient was approximately 0.4%/generation.      

Phytochrome control mechanisms in leaf expansion of Phaseolus vulgaris cv. Limburg.

Abstract. The effectiveness of a red-light pulse acting through phytochrome in inducing primary leaf expansion in 9-d-old etiolated bean ( Phaseolus vulgaris L. ev. Limburg) seedlings is strongly increased by a continuous far-red light (CFR) pretreatment. This increase in effectiveness of a red pulse is positively correlated with the time and the fluence rate of the CFR pretreatment. Escape from photoreversibility of this red pulse after the CFR pretreatment is extremely slow (more than 3 d). When a dark period is interposed between the end of the CFR pretreatment and the inductive red pulse the photoreversible part of the response to this pulse is highly dependent upon the photostationary state of phytochrome at the onset of the dark period.
The results give strong evidence for the synergistic activity of two components of phytochrome action during leaf growth induction, one of them acting via a very stable Pfr fraction.     

Parallel habitat specialization within the wolf spider genus Hogna from the Galápagos

Within most island archipelagos, such as the Galápagos, similar ecological gradients are found on geographically isolated islands. Species radiations in response to these ecological gradients may follow different scenarios being (i) a single habitat specialization event followed by secondary colonization of each ecotype on the different islands or (ii) repeated and parallel habitat specialization on each island separately. This latter scenario has been considered less likely as gene flow might hamper such ecotypic differentiation. At least for the Galápagos, the extent to which this process is involved in species radiations remains yet poorly understood. Within the wolf spider genus Hogna, seven species are described that can be divided into three different ecotypes based on general morphology and habitat preference i.e. species that inhabit the pampa vegetation in the highlands, species that occur in coastal dry habitats and one generalist species. Comparison of the species phylogeny based on one mitochondrial (COI) and one nuclear (28S) gene fragment convincingly demonstrates that ‘pampa’ and ‘coastal dry’ species evolved in parallel on the islands Santa Cruz and San Cristóbal. Despite the observation that allozymes analysis indicated that each species forms a distinct genetic cluster, phylogenetic divergence within these species complexes was very low and paraphyletic and most likely due to hybridization rather than incomplete lineage sorting, as demonstrated for the Santa Cruz species complex. This suggests that within‐island speciation occurred under low levels of gene flow. Species phylogeny in general did not follow the progression of island emergence as a molecular clock analysis suggested that island endemic species may have diverged after as well as before the emergence of the islands. This represents the first clear example of parallel and within‐island speciation because of habitat specialization on the Galápagos and that such divergence most likely occurred under historic gene flow.     

Evidence for autotetraploidy associated with reproductive isolation in Saccharomyces cerevisiae: towards a new domesticated species

Partial or whole‐genome duplications have played a major role in the evolution of new species. We have investigated the variation of ploidy level in a panel of domesticated strains of Saccharomyces cerevisiae coming from different geographical origins. Segregation studies and crosses with tester strains of different ploidy levels showed that part of the strains were well‐balanced autotetraploids displaying tetrasomic inheritance. The presence of up to four different alleles for various loci is consistent with a polyploidization mechanism relying on the fusion of two nonreduced meiospores coming from two S. cerevisiae strains. Autotetraploidy was also in accordance with karyotype and flow cytometry analyses. Interestingly, most bakery strains were tetraploids, suggesting a link between ploidy level and human use. The null or drastically reduced fertility of the hybrids between tetraploid and diploid strains indicated that domesticated S. cerevisiae strains are composed of two groups isolated by post‐zygotic reproductive barriers.     

Localization of a Mg2+-Activated ATPase in the Plasma Membrane of Trypanosoma cruzi1

The Wachstein and Meisel incubation medium was used to detect ATPase activity in epimastigote, spheromastigote (amastigote), and bloodstream trypomastigote forms of Trypanosoma cruzi. Reaction product, indicative of enzyme activity, was associated with the plasma membrane covering the cell body and the flagellum of the parasite. No reaction product was found in the portion of the plasma membrane lining the flagellar pocket. The plasma membrane-associated ATPase activity was not inhibited by ouabain or oligomycin, was detected in incubation medium without K⁺, was inhibited by prolonged glutaraldehyde fixation, and its activity was diminished when Mg²⁺ was omitted from the incubation medium. The Ernst medium was used to detect Na⁺-K⁺-ATPase activity in T. cruzi. No reaction product indicative of the presence of this enzyme was detected. Reaction product indicative of 5'-nucleotidase was not detected in T. cruzi. Acid phosphatase activity was detected in lysosomes. These results indicate that a Mg²⁺-activated ATPase is present in the plasma membrane of T. cruzi and that it can be used as an enzyme marker, provided that the mitochondrial and flagellar ATPases are inhibited, to assess the purity of plasma membrane fractions isolated from this parasite.