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The Journal of Heredity 1984:75(2):107-112
© 1984 The American Genetic Association 75:107-112

research-article

Genetically controlled differences in orientation in an interchange heterozygote of the German cockroach

Manuel Diez, Mary H. Ross, and Donald G. Cochran

Departmento de Genetica, Universidad Complutense de Madrid Madrid-3, Spain
Department of Entomology, Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061

Abstract

The interchange heterozygote, T(8;9)/8;9, in the German cockroach, Blattella germanica, is generally characterized by metaphase I orientations that fit a 2:1:l:+2 ratio of adjacent-1, alternate-1, adjacent-2, and alternate-2, respectively, resulting in overall directed disjunction. A new pattern, with orientation frequencies fitting a 2:1:1:2 ratio, occurred following a change of genetic background. This source was used to initiate selections from T/+ male matings with approximately 50 percent embryonic lethality, presumably maintaining the equality of alternate and adjacent orientations found in the new pattern (random disjunction). Males drawn randomly from the strain after 18 generations showed altered orientation patterns. The balance between adjacent-2 and alternate-1 was disturbed by an excess of adjacent-2 and a decrease of alternate-1 configurations. The change is attributed primarily to a nondistal chiasma that was found only in adjacent-2 rings. All males of the selected strain carried cells among which nondistal chiasmata occurred in adjacent-2 rings and the free bivalents. A similar phenomenon but with a somewhat different expression was found among approximately one-third of the males examined from another T(8;9)/8;9 stock (black body BI, backcross system). Interstrain variations in cellular frequencies of nondistal chiasmata, apparent differences in the position of the nondistal chiasma within adjacent-2 rings, and an elevated chiasma frequency in the selected strain are viewed as evidence of the diversity of genetic factors controiliing chiasma distribution in B. germanica. The orientation pattern in males of the selected strain is interpreted as basically representing homozygosity for random disjunction but with a secondary modification resulting from a nondistal chiasma.


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