GEOGRAPHICAL AND GENETIC STRUCTURE OF LIFE-HISTORY VARIATION IN MILKWEED BUGS (HEMIPTERA: LYGAEIDAE: ONCOPELTUS).

Life-history variation was investigated using crosses within and among the laboratory-bred descendants of six geographic samples of the large milkweed bug, Oncopeltus fasciatus. These samples spanned the species' range, from permanent (year-round) populations on tropical islands to seasonal middle-latitude populations found in temperate North America. The seasonal populations must be refounded each year by colonists from more southern populations. Marked differences in life-history traits (particularly in age at first reproduction, clutch size, and rate of egg production) were observed among the six population samples, with tropical-island and west-coast populations being the most distinct. In the eastern and central United States, there was a marked north-south difference in life history. Crossing experiments demonstrated a genetic basis for these differences. F and F hybrids from crosses between continental populations tended to have intermediate phenotypes. The similarity of the seasonal middle-latitude populations' life histories and the consistency of the distribution of life-history characteristics among populations (across years) may indicate that the north-south difference in life history is due to selection on these traits during the annual northward movement or that migrants represent a distinct genetic form of this species.