The Comstockiella system of chromosome behavior in the armored scale insects (Coccoïdea: Diaspididae)

Summary

The Comstockiella chromosome system occurs in the armored scale insects and the closely allied palm scales. During development of the males, the paternal chromosome set becomes heterochromatic and remains so until spermatogenesis. With the exception of one chromosome, the heterochromatic complement loses its differential aspect during early spermatogenesis and its members pair with their euchromatic homologues There is but one division during which the two components of each bivalent separate to opposite poles. Both division products form sperm.

One pair of chromosomes, the D pair, always shows differential behavior. The D pair usually does not form a bivalent. The heterochromatic homologue, D^H, divides equationally and is eliminated by anaphase lagging or telophase ejection; its daughter halves remain as pycnotic residues during the early phases of spermiogenesis. The euchromatic homologue, D^E, also divides equationally to contribute to both of the telophase nuclei. Compensation for the division of the D^E univalent may occur during either the early or late phases of spermatogenesis.

In some species the D pair is a fixed entity, analogous to the sex chromosomes in this regard. In other species, more than one pair may be elected to the D role, but only one at a time, and always the same one within each cyst.

Taxonomic evidence indicates the Comstockiella system was derived from the lecanoid system, previously known from the work of the Schraders and others. In the lecanoid system, the paternally derived heterochromatic set divides equationally, along with the euchromatic set, during the first spermatogenic division. During the second spermatogenic division, the two sets are segregated from each other. The two euchromatic derivatives form sperm while the heterochromatic derivatives persist for a while as pycnotic residues. Both the lecanoid and Comstockiella systems occur in some species often in the same testis, but only one of the two systems within any one cyst.

The discussion is devoted to an analysis of the mode of inheritance expected in the Comstockiella system and its evolutionary derivation. The Comstockiella system may have been derived in a step-by-step fashion from the lecanoid. The two systems differ by four processes which occur at spermatogenesis in the Comstockiella but not the lecanoid system; these are (1) deheterochromatization, (2) chromosome pairing, (3) compensation for the extra division of the D^E chromosome, and (4) lagging or ejection to eliminate the D^H chromosome.

In addition, the residual genetic effects of the heterochromatic set may have undergone considerable change before the lecanoid system could evolve into a Comstockiella. Once the evolutionary step were otherwise possible, mechanistic features would aid and abet the emergence of the new system even though it lacked immediate selective advantage.

The variable-D aspect of some examples of the Comstockiella system cannot be readily understood in terms of known examples of chromosome behavior; an admittedly highly speculative hypothesis is offered in an attempt to explain the situation.

The diaspidid system, in which the paternal chromosomes are eliminated at late cleavage, is believed on taxonomic grounds to have stemmed from the Comstockiella, and forms the final stage of the four-step evolutionary sequence. Necessary changes for the derivation of the diaspidid system from the Comstockiella are discussed.