Saturniid Hybrids

The giant silk moths in the family Saturniidae can be counted among the charismatic megafauna of the insects. Their richly patterned body coloration, sheer size, and exotic larvae make them popular among insect collectors, rearing hobbyists, and photographers, and they are even notable among the general public when they make their appearances to laypeople. Indeed, while relatively few in the scientific establishment seem to take the Saturniidae seriously as appropriate study organisms (a rant for another day), hobbyists in Europe and Asia, and to a lesser extent North America, have long bred and traded saturniids, observed and experimented with them informally, and occasionally documented their findings, sometimes in formal venues like low-profile peer-reviewed journals, but more often on personal websites, blogs, online fora, and other informal outlets.

One of the more intriguing aspects of the insect-keeping hobby I have been exploring lately is the hybridization of silk moth species. Bill Oehlke’s excellent website has a page that documents the outcomes of such experiments, many of which have had gorgeous results (check out the stunning moon moth hybrids involving Actias and the threatened Spanish moth Graellsia!). Many of these hybrid offspring are delicate creatures, making them doubly precious, and while many of these hybrids are undertaken by interested individual breeders, apparently there is a veritable European coalition of experienced and dedicated saturniid hobbyists that cross moths, distribute ova, and document each life stage of the hybrids as they develop.

I mean, just look at it! Look. At. It. A male from a cross between two of the most spectacular moon moth species, a male Graellsia isabellae and a female Actias maenas. Image by Eric Legrand, via Bill Oehlke.

On the surface, this all seems like a nice but perfectly useless Victorian gentlemen’s pastime, like collecting rare orchids or visiting exclusive gentleman’s clubs. However, saturniid hybridization experiments have yielded important scientific advances. The discovery by Walter Sweadner in the 1930s of a remote area in Idaho where two distinct populations of Hyalophora were interbreeding, for example, was the first to demonstrate that separate but related populations can hybridize and produce viable offspring in nature where their ranges overlap, even after they have already taken separate evolutionary paths. We now call these geographical areas ‘hybrid zones‘, and Sweadner’s studies helped established the foundations for our modern understanding of the role of interbreeding between populations during speciation. To this day, biologists continue to study hybridization in Hyalophora, a genus widespread in North America with regionally specialized but interbreeding populations.

Another hybrid, the oak tasar silk moth, Antheraea ‘proylei’, was developed as a multigenerational cross between domesticated A. pernyi males and wild A. roylei females in the 1970s, with the aim of boosting the Indian tussah silk industry in states with plentiful oak forest. Although the experiment was mainly a failure, A. ‘proylei’ is interesting from a genetic perspective because it is thought to be one of the few known natural cases of paternal mitochondrial DNA inheritance, with 98% of the mitogenome identical to A. pernyi (the males of which were backcrossed with the hybrids to found these lines). Also very unusual is that A. ‘proylei’ strains remain fertile indefinitely, despite mismatching chromosome numbers in the parent species (2n=98 for A. pernyi and 2n=62 for A. roylei), which is part of some convincing evidence that A. pernyi is merely a domesticated, genomically unstable descendant of A. roylei.

A male Polyphemus moth from the brood I raised during the summer of 2015, pooping on my fingers as moths are wont to do.

The whole point of this spiel was to introduce studies of giant silk moth hybrids as interesting, useful, and even foundational. I’ve been reading about these experiments because I want to attempt a little hybridization experiment of my own this summer, between my local strain of Antheraea polyphemus and a Japanese line of A. pernyi. Other hybrids have been produced among the unknown number of tropical and temperate Asian species, but these moths have all been rather same-y looking, reflecting the pretty uniform appearance of their parents. In the same vein, I have heard of hobbyists obtaining crosses of A. polyphemus with its close American relative A. oculea, and the resultant hybrid larvae and adults look almost exactly like regular A. polyphemus. Because the 4 New World species form a distant lineage separate from all their Old World cousins, and also because they differ in larval and adult characteristics, I am genuinely curious to see what hybrids of American and Asian species would look like, if they are viable at all.

Another reason why I’m interested in this project is because not many others have endeavored to do the same. Being that A. polyphemus is widespread in North America, A. pernyi is very common in Asian sericulture, and both have a history of use as study organisms in studies of insect development, diapause, and chemical communication, this is a cross that other hobbyists have surely already tried. However, I found fewer reports (at least, in English) on these attempts than I expected, and none described what interesting traits might be found in the offspring. The earliest attempt I could trace back was described in 1880 by an Alfred Wailly, who noted his lack of success:

Excerpt from Page 158 of Alfred Wailly’s article “Hybrids and Degeneracy” in the old British journal The Entomologist, Volume 13 (1880), from Google Books. I highly recommend Google Books and Biodiversity Heritage Library as databases when searching for old literature in the sciences.

On the other hand, Michael Collins & Robert Weast, in their 1961 book on the natural history of North American saturniids, state matter-of-factly that “Polyphemus freely crosses with [A. pernyi]. The larvae have been reared on oak.” And most notably, the Canadian lawyer Gary Botting, as a teenager in the late 1950s/early 1960s, carried out successful crosses* of Polyphemus moths (then named Telea polyphemus) with unspecified Asian moths (I assume either A. pernyi or A. yamamai), which supposedly convinced taxonomists to subsume the genus Telea as a North American branch of the genus Antheraea(EDIT: see below)

*Per Wikipedia, because I have been completely unable to find the paper trail for this project or even verify the experiment’s importance in influencing taxonomic decisions. (EDIT: see below)

These few uninformative and disagreeing accounts don’t provide results, data, or even pictures for a cross with such evident potential, and I could find no evidence on my own that anyone has attempted to mate an A. polyphemus with a single A. pernyi since Botting’s science fair project in the 1960s. With this in mind, I have written this blog post as a sort of New Year’s resolution to myself to start this experiment this summer. In the coming months I will continue to research how to obtain hybrid pairings and find collaborators, and if I can get a generation of A. polyphemus larvae going this year I’ll be able to start! I’m definitely looking forward to this journey and its results!

EDIT (1/3/2016): I looked into Charles Michener’s 1952 monograph on the genera of the New World Saturniidae. Throughout the monograph, Michener discusses Antheraea as a genus, though he confusedly names it under a subgeneric level (probably a typo) when he actually gets to describing it. More importantly, he synonymizes Telea, that is, lists it as an invalid name with lower priority than the name Antheraea, and concludes:

2016-01-03So even though Michener acknowledges that it was common practice at the time to use Telea polyphemus, he undoubtedly establishes Antheraea polyphemus as the correct name, nearly 10 years prior to Botting’s project.