By Arthur Dendy
Environmentally Induced Variations in Beetles
|1 "An Investigation of Evolution in Chrysomelid Beetles of the Genus Leptinotarsa," by William Laurence Tower. (Publications of the Carnegie Institution, Washington, 1906.)|
The modifications of the germ cells by virtue of which the offspring come to differ to a greater or less extent from their parents are, as we have seen, often attributed in large measure to permutations and combinations of different characters which take place in the sexual process (amphimixis) and the preceding nuclear reduction. It has long been suspected, however, that the germ cells themselves, apparently independently of the body in which they are enclosed, may be influenced by the environment to which an animal or plant is exposed, and the observations of Tower1 upon beetles of the genus Leptinotarsa may be referred to in this connection.
This observer considers that all permanent variations in these beetles, so far as can be discovered, arise in the germ cells and are in no wise the results of inherited somatic modifications. He attributes their appearance to the direct action of the environment upon the germ plasm and supports his views by a series of very interesting experiments. He subjected the parents to environmental stimuli of various kinds during the growth and maturation of their germ cells, and then, after the ova had been fertilized, allowed the development of the young to take place under normal conditions. The parents, having already reached their final state, were not themselves visibly affected by the stimuli, but a large percentage of the offspring showed surprising modifications which were strictly inherited. These modifications appear to be in no way adaptive. They seem to bear no relation to the nature of the stimulus which calls them forth and to be of no value to the organism in the struggle for existence.
We may cite one example of Tower's experiments. Four males and four females of Leptinotarsa decemlineata (the potato beetle) were exposed during the earlier part of the laying period (the eggs being matured and laid in successive batches) to extremely hot, dry conditions accompanied by low atmospheric pressure. The eggs were removed as soon as laid and reared under natural conditions. From 506 larvae thus reared 96 adult beetles were obtained, of which 82 were of a form known as pallida, 2 of a form known as immaculothorax, and the remainder unmodified. During the later part of the laying period the same parents were kept under normal conditions and yielded 819 eggs from which 61 normal beetles were obtained and none of the other forms, and these normal beetles continued to breed true for three generations, after which they were killed.
|1 See Chapter XIV.|
The two specimens of the immaculothorax form obtained in the earlier part of the experiment unfortunately died from disease, as also did all but two of the pallida. The remaining two, however, both being male, were crossed with normal females, yielding hybrid offspring with the decemlineata characters dominant, and these hybrids, breeding inter se, gave offspring which separated out in a characteristic Mendelian fashion1 into pallida, decemlineata and hybrids again. There can be no question therefore that the pallida characters, first due to modification of the germ cells by the action of changed environment, were strictly heritable.
It should be observed that the forms pallida and immaculothorax also occur occasionally, but rarely, in a state of nature as sports or mutations, a fact which suggests that sports or mutations in general may owe their existence to the apparently direct action of the environment upon the germ cells. It is, of course, possible, or even probable, that the change in the environment merely acts as a kind of liberating stimulus, which enables characters already latent in the germ cells to express themselves in the developing organism, which, under normal conditions, they are unable to do.