By Robert Deyes
Each year about 100 million Monarch butterflies from Canada and northeastern United States make their journey to the Mexican Sierra Madre mountains in an astonishing two-month long migration (Ref 1). They fly 2500 miles to a remote area that is only 60 square miles in size (Ref 1). No one fully understands what triggers this mass movement of Lepidopterans. But there is no getting away from the fact that this is a phenomenon that, as one review summed up, "staggers the mind", especially when one considers that these butterflies are freshly-hatched (Ref 1). In short, Monarch migrants are always "on their maiden voyage" (Ref 2). The location they fly to is home to a forest of broad-trunked trees that effectively retain warmth and keep out rain- factors that are essential for the Monarchs' survival (Ref 1).
With a four-inch wingspan and a weight of less than 1/5th of an ounce, it is remarkable that the Monarchs survive the odyssey (Ref 1). Making frequent stops for nectar and water, they fly approximately 50 miles a day avoiding all manner of predator. Rapidly shifting winds over the great lakes and scorching desert temperatures in the southern states provide formidable obstacles (Ref 1). Nevertheless the Monarchs' finely-tuned sense of direction gets most of them across.
It was not until 1975 that scientists first uncovered the full extent of the Monarch's migration (Ref 1). What has become clear since then is that only Monarchs travel such distances to avoid the "certain death of a cold winter". According to University of Toronto zoologist David Gibo, soaring is the key to making it to Mexico (Ref 1). Indeed flapping wings is about the most energy inefficient way of getting any where.
Other aspects of the Monarch's migration-linked behaviors, such as the reproductive diapause that halts energy-draining reproductive activity during its journey, continue to fascinate scientists worldwide (Ref 2). Both diapause and the 6-month longevity characteristic of Monarchs are caused by decreased levels of Juvenile Hormone which is itself regulated by four genes (Ref 2).
Exactly how Monarchs navigate so precisely to such a specific location is a subject of intense debate. One theory suggests that they respond to the sun's location, another that they are somehow sensitive to the earth's magnetic field (Ref 1). Recent molecular studies have shown that Monarchs have specialized cells in their brains that regulate their daily 'clock' and help keep them on course (Ref 3). Biologist Chip Taylor from the University of Kansas has done some remarkable tagging experiments demonstrating that even if Monarchs are moved to different locations during the course of their journey south, they are still able to re-orient themselves and continue onwards to their final destination (Ref 1).
A study headed by Stephen Rappert at the University of Massachusetts has elucidated much of the biological basis of the timing-component of Monarch migration (Ref 3). Through a process better known as time-compensated sun compass orientation, proteins with names such as Period, Timeless, Cryptochrome 1 and Cryptochrome 2 provide Monarchs with a well-regulated light responsiveness during both day and night (Ref 3). While Cryptochrome 1 is a photoreceptor that responds specifically to blue light, Cryptochrome 2 is a repressor of transcription, efficiently regulating the period and timeless genes during the course of a 24-hour light cycle (Ref 3). Investigations using Monarch heads have not only provided exquisite detail of the daily, light-dependent oscillations in the amounts of these proteins but have also revealed a 'complex relationship' of molecular happenings.
Indeed, the activities of both Cryptochrome 2 and Timeless are intertwined with at least two other timing proteins called 'Clock' and 'Cycle' (Ref 3). Preliminary results suggest that Period, Timeless and Cryptochrome 2 form a large protein complex, with Cryptochrome 2 being a repressor of Clock and Cycle activity. Cryptochrome 2 is also intimately involved with an area of the Monarch's brain called the Central Cortex that likely houses the light-dependent 'sun compass', so critical for accurate navigation (Ref 3).
Rappert's team have speculated that the Monarch's dual Cryptochrome light response system evolved into the single Cryptochrome systems found in other insects through a hypothetical gene loss event (Ref 3). Furthermore they have suggested that the dual Cryptochrome system itself arose through a duplication of an ancestral gene (Ref 3). Biologist Christopher Wills wrote of gene duplication as a 'rare occurrence' in which "an extra copy of a gene gets placed elsewhere in the genome" (Ref 4, p.95). Seen from an evolutionary perspective, these two gene copies are then "free to evolve separately...shaped by selection and chance to take on different tasks" (Ref 4, p.95).
While experiments have shown that transgenic Monarch Cryptochrome 1 can rescue Cryptochrome deficiency in other insects such as fruit flies, what still remains elusive is how exactly gene duplication could have lead to two proteins with such widely-differing functions as those found in the two Monarch Cryptochromes. Indeed biochemist Michael Behe has been instrumental in revealing the explanatory insufficiencies of terms such as gene duplication and genetic shuffling within the context of molecular evolution. As Behe expounded:
"The hypothesis of gene duplication and shuffling says nothing about how any particular protein or protein system was first produced- whether slowly or suddenly, or whether by natural selection or some other mechanism...In order to say that a system developed gradually by a Darwinian mechanism a person must show that the function of the system could "have formed by numerous, successive slight modifications"...If a factory for making bicycles were duplicated it would make bicycles, not motorcycles; that's what is meant by the word duplication. A gene for a protein might be duplicated by a random mutation, but it does not just "happen" to also have sophisticated new properties" (Ref 5, pp.90, 94)
When it comes to supplying a plausible mechanism for how gene duplication and subsequent natural selection led to two distinctly functioning Cryptochromes and how these then integrated with other time-regulatory proteins in Monarch brains, there is a noticeable absence of detail. Each successive slight modification of a duplicated gene would have had to confer an advantage, for selection and chance to get anywhere. Furthermore the newly duplicated Cryptochrome would have had to have become successfully incorporated into a novel scheme of daylight processing for migration patterns to begin.
Evolutionary biology must move beyond its hand-waving generalizations if it is to truly gain the title of a rigorous scientific discipline. In the meantime, protein systems such as the Monarch's Cryptochromes will continue to challenge what we claim to know about evolutionary origins.
1. NOVA: The Incredible Journey Of The Butterflies, Aired on PBS on the 27th January, 2009, See http://www.pbs.org/wgbh/nova/butterflies/program.html
2. Haisun Zhu, Amy Casselman, Steven M. Reppert (2008), Chasing Migration Genes: A Brain Expressed SequenceTag Resource for Summer and Migratory Monarch Butterflies (Danaus plexippus), PLoS One, Volume 3 (1), p. e1345
3. Haisun Zhu, Ivo Sauman, Quan Yuan, Amy Casselman, Myai Emery-Le, Patrick Emery, Steven M. Reppert (2008), Cryptochromes Define a Novel Circadian Clock Mechanism in Monarch Butterflies That May Underlie Sun Compass Navigation, PLoS Biology, Volume 6 (1), pp. 0138-0155
4. Christopher Wills (1991), Exons, Introns & Talking Genes: The Science Behind The Human Genome Project, Oxford University Press, Oxford UK
5. Michael Behe (1996), Darwin's Black Box, The Biochemical Challenge To Evolution, A Touchstone Book Published By Simon & Schuster, New York
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