Rapid diversification of sexual signals in Hawaiian Nesosydne planthoppers (Hemiptera: Delphacidae): the relative role of neutral and selective forces.

Changes in sexual signals have the potential to promote rapid divergence and reproductive isolation among populations of animals. Thus, identifying processes contributing to variation in signals is key to understanding the drivers of speciation. However, it is difficult to identify the processes initiating changes in signals in empirical systems because (1) the demographic history of populations under study is usually unclear, and (2) there is no unified hypothesis-testing framework for evaluating the simultaneous contribution of multiple processes. A unique system for study in the Hawaiian Islands, the planthopper species Nesosydne chambersi, offers a clear demographic context to disentangle these factors. By measuring variation in male vibratory sexual signals across different genetic populations on the island of Hawaii, we found that that multiple signal traits varied significantly between populations. We developed a mixed modelling framework to simultaneously test competing hypotheses about which processes contribute to changes in signal traits: genetic drift, sensory drive or reproductive character displacement. Our findings suggest that signal divergence proceeds along different axes for different signal traits under the influence of both neutral and selective processes. They are the first, to our knowledge, to document the relative importance of multiple processes on divergence in sexual signals.

The fate of severed corticospinal axons.

The potential for regeneration of severed corticospinal axons was examined by labeling these axons with horseradish peroxidase following thoracic spinal cord transections in mice. Shortly after severance, the proximal ends of corticospinal axons formed terminal bulbs that persisted for weeks and were associated with axonal retraction. There were few signs of corticospinal axonal sprouting or elongation. By 2 months after injury, corticospinal axons near the transection site showed an increased number of probable labeled terminals in the adjacent gray matter. These new terminals may contribute to the persistence of many corticospinal axons near the injury site long after a spinal cord transection.

Sequence of electromyographic abnormalities in stroke syndrome.

In 20 stroke patients who were examined by repeated electromyography, fibrillation potentials and positive waves were noted as early as seven to ten days after the stroke and gradually disappeared as volitional potentials and spasticity appeared. This sequence of events occurred first in the antigravity muscles, then in their antagonists and finally in the most distal muscles. We hypothesize that the abnormal irritability is a consequence of the loss of the neurotrophic influence on the muscle fiber after the stroke.