STAGES IN THE ORIGIN OF VERTEBRATES: ANALYSIS BY MEANS OF SCENARIOS

Vertebrates lack an epidermal nerve plexus. This feature is common to many invertebrates from which vertebrates differ by an extensive set of shared-derived characters (synapomorphies) derived from the neural crest and epidermal neurogenic placodes. Hence, the hypothesis that the developmental precursor of the epidermal nerve plexus may be homologous to the neural crest and epidermal neurogenic placodes. This account attempts to generate a nested set of scenarios for the prevertebrate-vertebrate transition, associating a presumed sequence of behavioural and environmental changes with the observed phenotypic ones. Toward this end, it integrates morphological, developmental, functional (physiological/behavioural) and some ecological data, as many phenotypic shifts apparently involved associated transitions in several aspects of the animals. The scenarios deal with the origin of embryonic and adult tissues and such major organs as the notochord, the CNS, gills and kidneys and propose a sequence of associated changes. Alternative scenarios are stated as the evidence often remains insufficient for decision. The analysis points to gaps in comprehension of the biology of the animals and therefore suggests further research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72629/1/j.1469-185X.1989.tb00471.x.pd

The membrane potentials of Tetrahymena vorax

1. 1. Tetrahymena in a solution containing 1 mM CaCl2, 1 mM KCl, 1 mM TrisHCl pH 6.8, and 0.1 mM EDTAKOH had a resting potential of -28.7 ± 2.1 mV and a membrane resistance of 44.1 ± 2.6 MΩ. 2. 2. A 10-fold increase in external K+ led to a 28.3 mV change, and a 10-fold change in Ca2+ led to a 9.7 mV change in resting potential. 3. 3. Spontaneous hyperpolarizations were observed. These were of 150 msec duration, 12 mV magnitude and had a rate of change of potential of 220 mV sec-1. 4. 4. Spontaneous depolarizations were observed. There were of two main types: (a) the fast type had a maximum rate of rise of potential of 500 mV sec-1 with the spike peaking at + 10 mV. The depolarization phase lasted 100 msec and the repolarizing phase 400 msec. The fast spikes had an all-or-none characteristic and a threshold between 3 and 7 mV; and (b) the slow depolarizations did not overshoot 0 mV, were 15 mV in magnitude, with a rate of rise of potential of 50 mV sec-1. The time course was variable. Both fast and slow depolarizations could occur in the same cell. 5. 5. Following spikes, sustained depolarizations (plateaus) were observed in sodium free solutions. They had a variable duration and were usually at 0 mV membrane potential. 6. 6. Electrical stimulation elicited the fast type of depolarization in 80% of Tetrahymena. The response had an all-or-none characteristic and the threshold was 0.25 nA. 7. 7. The electrophysiological properties of Tetrahymena appear similar to but not identical with those of Paramecium