TfTE GENETICS OF HABROBRACON JUGLANDIS ASHMEAD 



rathsr that the condition of the chromatin de- 

 termines cell sensitivity. 



The extreme and unexpected sensitivity of 

 metaphase I in Habrobracon oocytes has caused 

 some investigators to suggest that the effect 

 is "physiological" and not the result of direct 

 chromosome change. It has been found that hap- 

 loid (male) larvae are more sensitive to irra- 

 diation than diploid (female) larvae of Habro- 

 bracon, as expected if chromosome change is re- 

 sponsible. 



Eggs treated in metaphase I and in prophase 

 I all have chromosomes and fragments , when pres- 

 ent, distinct in outline in stages follov/ing 

 oviposition and no fusion bridges have been 

 seen. Absence of a low dose threshold for in- 

 jury and absence of a high one for clumping and 

 retardation of meiosis and early cleavage fur- 

 nish additional evidence against any serious 

 "physiological" effect in this stage. Since both 

 metaphase I and prophase I eggs, after their re- 

 spective lethal doses, present the same pattern 

 in regard to degree of development before death, 

 causes of death would appear to be of the same 

 nature for each. 



The linear proportionality between dose and 

 hatchability as well as the other facts about 

 metaphase I oocytes outlined abo/e can be ex^ 

 plained by the folio v; in g hypothesis (A. R. 

 Whiting, lQ45a). At the time of treatment, dy- 

 ads are unier tension since spindle fiber re- 

 gions have started toward the poles while ends 

 are still held in contact by chiasmata. When a 

 hit breaks a pair of chromatids (a dyad) proxi- 

 mal to a chiasma the parts will separate, too far 

 to rejoin but not far enough for the fracture to 



152 



