THE ELIMINATION OF CHROMOSOMES IN THE MEIOTIC DIVISIONS OF BRACHYSTETHUS RUBROMACULATUS DALLAS

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Abstract
The 4th lobe in all testes of the pentatomid B. rubromaculatus shows an aberrant meiosis of a very definite character. At metaphase the X and the Y take a normal position in the spindle, but the autosomal tetrads that have gathered in a clump in late diakinesis are shunted laterally out of the middle region of the cell, away from the polar axis. The 2 sex chromosomes divide equationally in this 1st division while the autosomal aggregate passes unaltered to either pole. In the 2d division the X and Y again behave as in normal meiosis and separate to opposite poles. The autosomal aggregate, if present, again passes undivided to either pole. Since the sex chromosomes behave normally throughout, and the aggregate of autosomes remains unchanged during both divisions, the following spermatids are produced: X; Y; X + autosomes; Y + autosomes. The last 2 types form abnormally large but otherwise normal sperms. It is more than likely that they do not function directly in the genetics of the species, but it is quite possible that in the polyspermic eggs they furnish unusually large quantities of nucleoproteins which are utilized by the developing embryo. The evolution of the abnormal lobe of the testis may thus confer an advantage on the species through its effect on the embryonic stages.