Mutational response of Habrobracon oocytes in metaphase and prophase to ethyl methanesulfonate and nitrogen mustard.

HITING (1945), working on the parasitic wasp Habrobracon, developed a timing method that distinguished oocytes irradiated in the first meiotic metaphase (metaphase I) from those irradiated in the first meiotic prophase (prophase I) . She used embryo lethality of X-irradiated eggs from diploid virgins, which measured mostly dominant lethality, and found that metaphase I oocytes are approximately 20 times as sensitive to X-radiation as prophase I oocytes. When methods for determining recessive lethal frequency for the entire genome of Habrobracon were established (HEIDENTHAL 1952), it was found that metaphase I is also 20 times as sensitive when recessive lethality is the criterion (ATWOOD, VON BORSTEL and WHITING 1956; WHITING and MURPHY 1956). The egg of Habrobracon has such a thin chorion that the stage of embryonic death can be recorded when hatchability is scored (VON BORSTEL and REKEMEYER 1959). With stage of death data available, dominant lethality can be partitioned into different types. The actions of mutagenic agents can be compared with respect to frequencies of early death, which is believed to be associated with mitotic failure (VON BORSTEL 1960, 1961a,b), and later deaths, which derive from a loss of chromosomes or their parts (VON BORSTEL and REKEMEYER 1959). Nitrogen mustard [methyl bis ( p-chloroethyl) amine hydrochloride] has been used on the different stages of oogenesis in Habrobracon, and a 20-fold difference in sensitivity between metaphase I and prophase I for embryo lethality, like that of X-radiation, obtains with this agent ( VON BORSTEL 1955a). Neither recessive lethality nor stages of death were scored accurately at that time, but it was noted that the frequency of early deaths induced by nitrogen mustard is higher, relative to the mean lethal dose, than that induced by X-radiation (cf. VON BORSTEL 195513). Nitrogen mustard is an alkylating agent which reacts with purine and pyrimidine bases (LAWLEY 1957; LAWLEY and WALLICK 1957; BROOKES and LAWLEY 1960); since the molecule has two reactive arms, cross-linking of polynucleotide chains of deoxyribonucleic acid (DNA) has also been suggested as a source of its mutagenic and/or lethal action (GOLDACRE, LOVELESS and ROSS 1949; LOVELESS 1959; BROOKES and LAWLEY 1961 ) . The mutagenic action of ethyl methanesulfonate (EMS) has been observed in a number of organisms, including bacteriophage (LOVELESS, 1958, 1959), E. coli