Monarch – 5th Instar
Now in the 5th instar, 53.9mm long on day 14 post hatch.
Many people are familiar with the incredible migration of the monarch butterflies from all over North America to Mexico. Coupled with it’s beautiful and distinctive coloration the migration is what makes this butterfly as well known and appreciated.
According to recent DNA analysis by Smith et al. in 2005, there are actually two sub species of Danaus plexippus. The nominate, migratory species we have featured here is Danaus plexippus plexippus and there is a non-migratory species found from Georgia west to Texas south through the Caribbean and Central America to the Amazon Basin named Danaus plexippus megalippe. South of the Amazon is separate species of monarch (D. erippus) that Smith et al. estimated split form the D. plexippus only 2mya.
The migratory subspecies essentially has two separate life cycles. The migrants overwinter in Mexico (where there are now significant anthropogenic pressures on their overwinter site) then in the spring mate and return to the southern part of the United States. They lay their eggs on the spring milkweeds in the south then die. The eggs that survive and make it to adults continue the migration northwards, each generation living as adults only for three to four weeks.
In late August and September cooler nights signal a change in the cycle. The last generation, now 3-5 generations removed from the spring’s migration group, do not complete reproductive development and will not become reproductively active until the following spring. This group has a significantly lower activity in a key hormone for development – “Juvenile Hormone” (JH). The JH deficiency stops development, and significantly extends the butterfly’s longevity, from a few weeks in summer butterflies to several months in the migrants. Experimenters have manipulated JH in non-migratory adult butterflies with the predicted changes in reproductive activity and longevity.
One of the most remarkable things about the migration of monarchs is that each year the migrants are on their maiden voyage. The entire migration route and cues are genetically wired. In their paper “Chasing Migration Genes: A Brain Expressed Sequence Tag Resource for Summer and Migratory Monarch Butterflies (Danaus plexippus)“, Haisun Zhu, Amy Casselman and Steven M. Reppert created a library of expressed sequence tags (EST’s) for D. plexippus plexippus. An expressed sequence tag is a unique sub-sequence of DNA in the encoding region of a gene which can be used to positively ID the gene and help map the gene to a specific chromosome location.
The researchers created the library with samples from both non-migrating (summer) and migrating (fall) butterflies. They estimate that their library could represent about 52% of the encoding potential of the D. plexippus genome. Throught the library they discovered that three genes (allatotropin, juvenile hormone acid methyltransfersase, and takeout) are responsible for increased JH activity in summer butterflies. The library also contained several genes identified with the circadian clock which governs and guides the monarch migration and a new allele of an Na+/K+ ATPase in summer only monarch which should confer more resistance to the toxic effects of milkweed than that reported previously.
- Lepidoptera (Butterflies and Moths)
- Danaidae (Milkweed Butterflies
- Danaus plexippus
Haisun Zhu, Amy Casselman, Steven M. Reppert, Dawn Albertson (2008). Chasing Migration Genes: A Brain Expressed Sequence Tag Resource for Summer and Migratory Monarch Butterflies (Danaus plexippus) PLoS ONE, 3 (1) DOI: 10.1371/journal.pone.0001345