No abstract

The opossum, Monodelphis domestica, is born very immature but crawls, unaided, with its forelimbs (FL) from the mother's birth canal to a nipple where it attaches to pursue its development. What sensory cues guide the newborn to the nipple and trigger its attachment to it? Previous experiments showed that low intensity electrical stimulation of the trigeminal ganglion induces FL movement in in vitro preparations and that trigeminal innervation of the facial skin is well developed in the newborn. The skin does not contain Vater-Pacini or Meissner touch corpuscles at this age, but it contains cells which appear to be Merkel cells (MC). We sought to determine if touch perceived by MC could exert an influence on FL movements. Application of the fluorescent dye AM1-43, which labels sensory cells such as MC, revealed the presence of a large number of labeled cells in the facial epidermis, especially in the snout skin, in newborn opossums. Moreover, calibrated pressure applied to the snout induced bilateral and simultaneous electromyographic responses of the triceps muscle in in vitro preparations of the neuraxis and FL from newborn. These responses increase with stimulation intensity and tend to decrease over time. Removing the facial skin nearly abolished these responses. Metabotropic glutamate 1 receptors being involved in MC neurotransmission, an antagonist of these receptors was applied to the bath, which decreased the EMG responses in a reversible manner. Likewise, bath application of the purinergic type 2 receptors, used by AM1-43 to penetrate sensory cells, also decreased the triceps EMG responses. The combined results support a strong influence of facial mechanosensation on FL movement in newborn opossums, and suggest that this influence could be exerted via MC.

No abstract

Three strains (L3, L5 and L8) of White Leghorns reproduced by random mating in a closed population for over 20 yr were subjected to family selection for annual egg production. The annual egg production period was divided into various part period segments with their corresponding residual egg production records. Combined estimates of heritabilities and genetic correlations were calculated for each segment of the production curve. Relative efficiency of the genetic progress expected in the annual and residual records using part record vis-a-vis complete part record was estimated.For each part period segment heritabilities were the same -whether they were estimated from egg number or from percent production -and showed very high values when estimated from a segment that includes the 21 st wk. Heritabilities of the residual records were low for the L3 and moderate for the L5 and L8 strains. Percent production or egg number of the complete part record showed a very high genetic correlation with annual egg production in all strains, a high genetic correlation with residual record in the L8 strain, and a low genetic correlation with residual record in the L3 and L5 strains. A very high annual relative selection efficiency could be obtained by using for selection the percent production of complete part record, but the persistency of layers could be increased by using for selection the percent production of other partial records. (

No abstract