South Strains Research Articles

Reproductive Patterns Drive the Gene Flow and Spatial Dispersal of Euschistus heros (Hemiptera: Pentatomidae).

Euschistus heros (Hemiptera: Pentatomidae) has two allopatric strains with a hybrid zone in central Brazil. Asymmetric dispersal and gene flow between these strains of E. heros have been observed, where the South strain (SS) moves more quickly to the northern regions of the country than the North strain (NS) to the southern areas. In addition, SS generally has a bigger body size and presents dark brown coloration, and NS is usually smaller in size and presents light brown coloration. Here, we studied the reproductive behavior and tested for the presence of assortative mating and reproductive barriers between the two allopatric strains of E. heros. Nonrandom mating was observed in the SS strain based on mating choice trials and the reproductive isolation indexes. SS females and males prefer to mate with their co-specific (same strain) partner, while NS insects showed no mating preference. The insect's pronotum width was positively associated with the mating choice suggesting size-assortative mating in E. heros. Reciprocal crosses between strains yielded similar reproductive outputs when compared with pure strain crosses, suggesting similar fitness of hybrid pure strains. The asymmetric gene flow in the hybridization zone that favors SS seems to be associated with the reproductive behavior of the species, which favors the typical phenotype found in the SS populations.

The isolation and inheritance of dieldrin resistance in the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae)

AbstractFour geographical strains of Ceratitis capitata (Wiedemann) were subjected as third-instar larvae to mass selection with dieldrin. Two strains, SOUTH and COSRIC, responded well but continued to show substantial heterogeneity in tolerance to the insecticide after 21 and 25 generations of selection, respectively. The technique was altered to single family sib selection, resulting in the isolation from the SOUTH strain of a dieldrinresistant strain (73) showing a more homogeneous response to the insecticide. By the same technique, a dieldrin susceptible strain (72/3) was isolated from the COSRIC strain. A decline in resistance was observed in the 73 strain between generations three and four, but the level subsequently increased again, without selection, reaching 68-fold in generation seven. It remained high for the next two years. No significant change was observed in susceptibility to dieldrin of strain 72/3; it remained highly susceptible. Resistance to solutions of technical dieldrin (96%) was shown to be mainly under the control of a single major autosomal gene, intermediate in dominance but with a modifying effect of the genetic background on the expression of the resistance gene. The resistance heterozygotes, segregating in backcrosses to susceptible individuals, were shown to be more resistant than expected, while in backcrosses to resistant ones they were more susceptible. A dose of 11 p.p.m. dieldrin was found to discriminate almost completely between heterozygous and homozygous-susceptible individuals among backcross progeny. In tests using a purer form of the insecticide (99·7%), a similar pattern of resistance was observed. The implications for genetic sexing in C. capitata are discussed.