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Any geneticists on board?

There are two reasonably distinct size morphs in C. bottae, a dwarf form found at the southern extreme of the species' range in Calif. and the large morph that exists in the remainder of the species' range in North America from Tulare county, Calif. northward and eastward.

Dwarf C. bottae produce small neonates that range up to about 8 5/8" with a mean length between 7 1/2 to 8" Don't have a large enough sample from many regions of the large morph but locally in Oregon and information on litters from other regions indicate that neonates are generally 9" or above in length. The mean length for the local large morph females is slightly over 10".

In 1996 and 1999 I crossed dwarf males x large morph females. The size of resultant neonates were within the large morph neonate lengths. I completed a similar cross this year with the same results.

I should mention that one of the defining features between the two morphs is that maximum lengths of the dwarf form females is around 22" whereas all population of the large morph for which I have a reasonable sample have had females of 25" or longer.

A hybrid female from the 1996 cross became sexually mature during last summer just before her 7th birthday and at around 22 1/2" in length. By last Oct. she attained the robust condition necessary to carry a litter this year. This spring I backcrossed her to two dwarf males.

I had formed a hypothesis that what has kept the two morphs distinct where they must intergrade in the southern Sierras is that large morph females produce large ova resulting in large neonates and dwarf females produce smaller ova that result in much smaller neontates. Since ova are developing well before mating occurs and then the sperm are stored until ovulation, I have, perhaps erroneously, discounted influence of the parent males.

The hypothesis was simply conjecture except for the results of the dwarf male x large morph female crosses which have produced large morph neonates.

The hypothsis just bit the dust three days ago when the hybrid female produced her litter of one slug and 5 young--- all of which were dwarf.

Is there anyone that knows of a genetic explanation? Is anyone aware of 'delayed maternal inheretance'? After fertilization, could factors in the male sperm affect the eventual size of the developing embryos? it that were the case, then just by change alone once in awhile one could expect the results obtained. Yet that would suggest simple Mendelian inheretance and it would have been my guess that polygenic inheretance would be involved with size factors.

Yet despite just this one sample, the outcome suggests some sort of discrete pattern of inheretance.

In 2006 when this female is again in reproductive condition, do I make the same backcross to hopefully verify current results and increase my sample or should I make the reciprocal backcross to a large morph male?

Stumped! Richard F. Hoyer

Hi, thanks for sharing the results of your very interesting breeding experiments.

You wrote:

"There are two reasonably distinct size morphs in C. bottae, a dwarf form found at the southern extreme of the species' range in Calif. and the large morph that exists in the remainder of the species' range in North America from Tulare county, Calif. northward and eastward."

The situation is slightly more complicated than that. According to the mtDNA data of Rodriguez-Robles et al., all populations of the Rubber boa share the same common ancestor. Sometime in the distant past, this ancestor gave rise to a southern and a northern lineage of rubber boas. A bit more recently the northern lineage further diverged into 2 sublineages, which Rodriguez-Robles et al. call the Northwestern and Sierra Nevada subclades.

The southern lineage is comprised entirely of dwarf morph populations, as you have said before. The problem arises when we examine the northern lineage. One of the subclades (the Northwestern subclade) consists entirely of large morph snakes, according to a reading of your past posts. The other subclade, namely the Sierra Nevada subclade, however, consists of a mixture of large and small morph populations, with the small morphs confined to the area south of Tulare County, California, again according to your past posts, and the large morphs to the north of these small morph snakes. Because of the proximity of small morph Sierra Nevada subclde snakes to the apparently ancestral Southern California populations, it is most logical to presume that these small morphs from different lineages most closely resemble each other and their common ancestor. In other words, the ancestral form of rubber boas is the small morph. The small morph Sierra Nevada snakes and the Southern California snakes have thus largely remained unchanged from their common ancestor morphologically. The large morph(s), on the other hand, probably evolved more recently.

Looking back at Rodriguez-Robles' Fig. 4, one can see that the small morph snakes from Kern County last shared a common ancestor quite recently with the large morph Sierra Nevada snakes to the north. In fact, these two morphs share a common ancestor more recently with each other than either of them does with the Northwestern subclade. This sort of evolutionary relationship suggests several possibilities:

1). The last common ancestor of the Northwestern and Sierra Nevada subclades is a small morph population, and the large morph evolved independently twice, once in the Northwestern and once in the Sierra Nevada subclades.

2). The last common ancestor of the Northwestern and Sierra Nevada subclades is a large morph, and the southern populations of the Sierra Nevada snakes subsequently underwent evolutionary reversal and re-evolved dwarfism from a large morph ancestor.

3). As you have suggested in the past, there may have been gene flow between snakes of the Central Sierra Nevada and the Bay Area snakes via the “Trans-Valley leak.” Certainly such gene flow could have brought the genes coding for large morph from the Northwestern subclade to the Sierra Nevada populations, which originally consisted entirely of small morph populations.

Possibility #3 would suggest that the large morph evolved only once, which is certainly parsimonious. However, the mtDNA data shows no evidence of such a “Trans-Valley leak.” That does not mean that it did not happen, just that there is no proof that it did. Possibility number 2 requires a reversal, which is of course not rare but perhaps a little less likely than possibility #1. Possibility number one is perhaps the most likely although one cannot completely rule out either possibilities 2 or 3.

Unfortunately I cannot help you out with the interpretation of the results from your breeding experiments, but they are interesting and they hopefully will stimulate further research into the origin and evolution of large morph(s) in the rubber boa. It is also unfortunate that Rodriguez-Robles et al.’s data cannot tell us with certainty whether the large morph evolved once or twice independently in the Northwestern and Sierra Nevada subclades. Please keep in mind that there is as yet no evidence to show whether there is one large morph or two independently evolved large morphs in the rubber boa. If the large morphs evolved independently, there is also the necessity to determine whether they are under the control of different genes (convergence) or whether they are under the control of the same genes (parallelism). The rubber boa is a very interesting species indeed.

Regards.

Reference
Rodriguez-Robles,Javier A., Glenn R. Stewart, and Theodore J. Papenfuss, 2001. Mitochondrial DNA-Based Phylogeography of North American Rubber Boas, Charina bottae (Serpentes: Boidae). Molecular Phylogenetics and Evolution, Vol. 18, No. 2, pp. 227–237

C. King:
Thanks for the added input.

But may I ask why cannot the original ancestor of all boa populations be the boas that now occur in the San Bernardino and San Jacinto Mts. and have remained virtually unchanged for millions of years?

Richard F. Hoyer

Richard F. Hoyer wrote:

"But may I ask why cannot the original ancestor of all boa populations be the boas that now occur in the San Bernardino and San Jacinto Mts. and have remained virtually unchanged for millions of years?"

That is a very good question. Evolutionary stasis is indeed a very well known phenomenon. Darwin, for example, recognizes its existence. He writes in his book "Origin" thusly:

'We will suppose the letters A to L to represent allied genera, which lived during the Silurian epoch, and these have descended from a species which existed at an unknown anterior period. Species of three of these genera (A, F, and I) have transmitted modified descendants to the present day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal line. Now all these modified descendants from a single species, are represented as related in blood or descent to the same degree; they may metaphorically be called cousins to the same millionth degree; yet they differ widely and in different degrees from each other. The forms descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families. Nor can the existing species, descended from A, be ranked in the same genus with the parent A; or those from I, with the parent I. But the existing genus F14 may be supposed to have been but slightly modified; and it will then rank with the parent-genus F; just as some few still living organic beings belong to Silurian genera. So that the amount or value of the differences between organic beings all related to each other in the same degree in blood, has come to be widely different."

Darwin therefore recognizes stasis in the genus he calls "F14" and he also suggests classifying species in "F14" and its ancestor "F" in the same genus. Darwin therefore not only advocates the recognition of new taxa on the basis of morphological disparity (unlike the cladists) but he also advocates the recognition of paraphyletic taxa (again unlike the cladists).

The rubber boa would appear to be a morphologically conservative species that has changed little over tens of millions of years. Its closest relative, Lichanura trivirgata, is quite similar morphologically to it, suggesting that both of these boas have remained largely unchanged over time, thus resembling each other and their common ancestor closely morphologically. The alternative is that both species of boas are quite different from what they used to be morphologically and they now look similar merely because they have evolved convergently to resemble each other because of adaptation to a similar mode of life. The second alternative is less likely since Arnold Kluge, on the basis of morphological similarity, argues that Calabaria is the closest relative of Charina and Lichanura. Either these 3 genera converged upon each other morphologically or they are morphologically conservative genera which resemble each other because of shared ancestral characters.

Since the Southern California snakes occupy a basal position in the mtDNA tree, they are thus closest to the common ancestor of all boas genealogically. That said, the mtDNA tree does show that the many extant populations of the San Jacinto and San Bernardino mountains share a common ancestor quite recently. It is as though the populations of boas suffered a catastrophic crash and these populations have only recently recovered and expanded their range from perhaps as small as a few localities or even a single locality.

Biologists like to point out that even though land vertebrates are descendants of a fish, none of the species of fishes that are living today is the direct ancestor of the land vertebrates. In the same way, none of the extant populations of San Bernardino and San Jacinto boas can be said to be the direct ancestor of the other populations of extant boas in the Sierra Nevada or the rest of this species' range. Biologists like to point out that all extant species are the descendants of a long extinct common ancestor. That said, the morphology of this extinct ancestor can often be inferred. It is indeed my inference that the common ancestor of all boas are, like the San Jacinto and San Bernardino mountain boas, small morph snakes that are also morphologically very similar to the living rubber boas occupying these mountain ranges.