A Re-examination of Reproductive Arrest in the Monarch Butterfly, Danaus plexippus.

Abstract

Migratory and overwintering monarch butterflies, Danaus plexippus, are observed in a non-­reproductive state classified as either reproductive diapause or oligopause. The stimuli that lead to this reproductive condition have been characterized as changes in photoperiod, declining host plant quality, and temperature (Goehring and Oberhauser 2002), and in another study simply as temperature (James 1982). This study was conducted to examine cool temperature as the stimulus for the induction of reproductive arrest and to correctly classify reproductive arrest as either reproductive diapause or oligopause. Reproductive arrest was studied using monarchs reared in the laboratory. Butterflies were allowed to fly, bask, and nectar freely within screened cages. Cages were kept in temperature controlled growth chambers. Oocyte presence and ovarian development score were used to determine reproductive status. The mean number of mature oocytes was dependent on temperature. Females exposed to a mean temperature of 15°C failed to develop mature oocytes during the course of the experiment. All females held in the moderate temperature treatment (17°C), did not develop any mature oocytes, but many immature oocytes were observed in 50% of the butterflies. Females held in the warm temperature treatment (25°C) developed mature oocytes. Females exposed to the cool temperature treatment three days post eclosion showed similar patterns. Temperature is the main factor controlling reproductive arrest in D. plexippus. A second experiment was conducted to classify reproductive arrest. Reproductive arrest was induced and maintained by exposing females to a constant 12°C temperature regime for either 4 or 8 days. These cold pretreated females did not retain ovarian dormancy after two days of exposure to 28°C temperatures. Pretreated females showed reproductive development similar to control females that were never exposed to 12°C. No observable refractory period was seen in cold-exposed females. Due to the lack of such a refractory period, reproductive arrest in the female monarch is better classified as oligopause, not reproductive diapause.