Isogenic grasshoppers: genetic variability in the morphology of identified neurons.

Although identified neurons are defined as constant and unique from animal to animal within the same species, we might expect to find phenotype variability in the number, morphology, and physiology of identified neurons if we examined them in enough sexually reproduced animals of the same species. Morphological variability and axonal abnormalities were previously encountered in a study of the large ocellar interneurons in 50 grasshoppers. Both the somata and arborizations in the brain of the ocellar interneurons from one or more ocelli can be stained easily and repeatedly with cobalt, so that one can routinely examine the morphology of large numbers of cells in large numbers of animals. In the present study, the large ocellar interneurons were examined in three types of animals: (i) animals from two different breeding populations and their offspring, (ii) the offspring within clutches from single mated pairs of known phenotypes, and (iii) isogenic animals from the parthenogenetic clones of single unmated females. The soma location, axonal pathway, location of arborizations, and fine branching patterns of 14 of the large ocellar interneurons were examined in 430 grasshoppers. The large ocellar interneurons were examined in 11 clones raised in nearly constant and identical conditions. In most of the clones, the morphology of interneurons L5L and L5R was normal; all of the 82 animals examined from clones 4, 7, 9, 10, 12, 13, 16, and 17 were normal. Clones 8 and 2 had high percentages of animals with axonal abnormalities of interneuron L5: 16 out of 24 animals (66%) in clone 8 and 37 out of 42 animals (88%) in clone 2. All of the ocellar interneurons except for L5 were normal in all of these animals. Thus, the abnormal development of at least one identified interneuron, L5, can occur with a high degree of genetic control and a high degree of specificity. The axonal abnormality of interneuron L5 was not the same in each of the 37 animals from clone 2, although common mistakes and patterns were detected.