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J M Dowd

Publications and source records attributed to J M Dowd.

7 recordsLinked to original sources
PubMed1995

Chloroplast gene sequence data suggest a single origin of the predisposition for symbiotic nitrogen fixation in angiosperms.

Of the approximately 380 families of angiosperms, representatives of only 10 are known to form symbiotic associations with nitrogen-fixing bacteria in root nodules. The morphologically based classification schemes proposed by taxonomists suggest that many of these 10 families of plants are only distantly related, engendering the hypothesis that the capacity to fix nitrogen evolved independently several, if not many, times. This has in turn influenced attitudes toward the likelihood of transferring genes responsible for symbiotic nitrogen fixation to crop species lacking this ability. Phylogenetic analysis of DNA sequences for the chloroplast gene rbcL indicates, however, that representatives of all 10 families with nitrogen-fixing symbioses occur together, with several families lacking this association, in a single clade. This study therefore indicates that only one lineage of closely related taxa achieved the underlying genetic architecture necessary for symbiotic nitrogen fixation in root nodules.

Base Sequence↗
PubMed1991

A comparison of 18s ribosomal RNA and rubisco large subunit sequences for studying angiosperm phylogeny.

Partial sequences of 18s rRNA were obtained for 2 gymnosperms and 12 angiosperms from a wide range of families and these were analyzed with 5 other published sequences to form a phylogenetic tree. Using 16 published sequences of the large subunit of rubisco (rbcL), also from a wide range of angiosperm families, another phylogenetic tree was derived and the two approaches were compared. Both phylogenetic trees gave good grouping within families but in neither case was there resolution of the branching order of major taxa. Superficially the long rbcL sequences (whose base composition was homogeneous among all species) seemed very promising, but analysis showed that a large proportion of the variation did not affect the amino acid sequence. Although silent substitution contained some phylogenetic information, at the level required to order major taxa, much of it was random and obfuscating. It was concluded that neither macromolecule alone was likely to yield a solution to the problem of angiosperm phylogeny and therefore that studies of both, at least, will be required. For this reason, a method was described for obtaining both DNA and RNA of good quality from the same preparation and which had been used successfully with a wide range of species including many with pungent leaves.

Base Sequence↗
PubMed1986

Temporal coordination of arm movements in early infancy: do infants move in synchrony with adult speech?

The purpose of the present study was to measure the degree of interdependence in timing (a) between infants' right and left arm movements, and (b) between movements of both arms and the onsets of stressed vowels in tape-recorded infant-appropriate speech. Based on previous work concerning closely related phenomena, we hypothesized that both forms of interdependence should be present to a high degree, even very early in infancy. Movements of 2 male and 4 female infants were analyzed at 2 and 10 weeks of age. Videotaping was conducted under 3 conditions: (a) quiet, (b) mother-infant interaction, and (c) presentation of a repeating tape loop containing natural sounding, infant-appropriate speech. Movement events were analyzed using a method of computer-aided 3-dimensional cinematography. A computer algorithm located change-in-direction movement events based on the 3-D coordinates. Lag sequential analysis was used to test for temporal interdependence. No evidence was found for interdependence between right and left arm movements, or between speech events and arm movements. The present findings have important implications for our understanding of early coordination and the regulation of mother-infant interaction. The utility of the movement-scoring methodology is also discussed in terms of its potential application to the study of early coordination development.

Female↗
PubMed1984

A developmental look at an auditory illusion: the precedence effect.

Infants aged 2 and 6 months were tested with the precedence effect, an auditory phenomenon involving sound localization. Each infant was tested with two types of stimuli: sound from a single loudspeaker and precedence-effect sounds produced by the same sound put through two loudspeakers, with one output leading the other by 7 msec. Older infants localized precedence-effect stimuli as they did single-source stimuli, indicating that they perceived this phenomenon as expected. Two-month-olds turned their heads toward single-source sounds, but did not localize precedence-effect sounds, suggesting that that more difficult perceptual task had not been achieved at this age. In general, head-turning toward sound proved far more difficult to elicit in younger infants. A click train was ineffective, but a tape-recorded human voice elicited above-chance low-level turning. The developmental changes in auditory behavior are discussed in terms of the rapid growth of the auditory cortex.

Arousal↗
PubMed1981

Newborns' orientation toward sound: possible implications for cortical development.

The precedence effect is an auditory illusion produced by presenting the same signal through 2 loudspeakers, with 1 leading the other by several milliseconds. Adults perceive a sound localized exclusively on the leading side and directionally equivalent to a single source sound. Because the precedence effect is thought to involve cortical functions, newborns were expected not to respond with directional head turning toward these sounds. Newborns were presented with a tape-recorded rattle sound produced in 3 ways: through a single loudspeaker located right or left, through both loudspeakers with 1 onset leading the other by 7 msec, and control stimuli in which both loudspeakers sounded simultaneously, resulting in an apparent center location of the sound. Newborns turned toward the single source sound, but neither to precedence effect stimuli nor control stimuli. These results were related to maturation of the auditory cortex.

Auditory Cortex↗