RADIATION ANALYSIS OF A LECANOID GENETIC SYSTEM

Abstract

IN 1921, SCHRADER first reported the unusual cytological system in a mealy bug Pseudococcus nipae (Mask.). Further work by SCHRADER (1923a) and HUGHES-SCHRADER (1935) resulted in an exact characterization of a system of chromosome behavior which has been named “lecanoid.” These and other studies are discussed in HUGHES-SCHRADER’S ( 1948) comprehensive review of coccid cytology. One set of chromosomes undergoes facultative heterochromatization in male embryos and becomes excluded from the genetic continuum at spermatogenesis. Although a striking departure from the more conventional forms of chromosome behavior, the lecanoid system is not a passing accident but has sustained quite considerable evolution. It occurs throughout a series of diverse families including the primitive mealy bugs (Pseudococcidae) , the cochineal dye insects, the lac insects, the soft scales (Lecaniidae) , the asterolecanoids, and the conchaspids as well as the most specialized coccids, the armored scale insects (Diaspididae) in which it occurs as a dependent system (HUGHES-SCHRADER 1948; BROWN 1959). In the lecanoid system both males and females are diploid. One haploid set becomes heterochromatic at the blastula stage in the male embryos but otherwise the chromosomes of the two sexes are not distinguishable. The heterochromatic set divides synchronously with the euchromatic but forms a conspicuous chromocenter in the resting nuclei. Chromosome behavior is usually the same in soma and germ line; however, according to recent observations (BROWN, unpublished; NUR, unpublished) , heterochromatization is frequently absent from certain polyploid nuclei and may be restricted to the germ line in a few species. At spermatogenesis (Figure 1 ) , the first mitosis is equational for both types of chromosomes while the second is segregational, the heterochromatic set moving away from the euchromatic on a monopolar or highly asymmetric spindle. Only the euchromatic derivative forms sperm; the heterochromatic entity forms a highly pycnotic residue which is maintained during much of spermiogenesis. Chromosome comportment in the female is normal during ontogeny and oogenesis. The presence of chiasmata in the bivalents may be taken to mean that recombination occurs by crossing over as well as by random assortment. SCHRADER and HUGHES-SCHRADER (1931) believed the lecanoid system to be a stepping stone toward true haplodiploidy. The lecanoid male transmits only a single set of chromosomes and thus will breed as a haploid if this set were derived exclusively from either parent. SCHRADER and HUGHES-SCHRADER (1931) suggested that the heteropycnosis of one set of chromosomes indicates its genetic inertness; the male would therefore be a virtual haploid. HUGHES-SCHRADER (1948) further suggested that the heterochromatic set is of paternal origin. The present study was undertaken to test the validity of these two hypotheses by means of respanse to irradiation as suggested by P. W. WHITING'S (1937, 1938) studies of the true haplodiploid system of Habrobracon.