I really no longer think that ball pythons that are homozygous for the spider allele show embryonic lethality.

Why?

Not because there are not examples of pleiotropy in ball pythons or reptiles…..the superwoma (the pearl) demonstrate that genes influencing bp skin color can influence other body systems.

Not because there are no examples of homozygous lethal alleles in any snake species. The Jaguar gene in Carpet pythons appears to be homozygous lethal. “Supers” die shortly after birth. See http://www.naturalselectionreptiles....hersnakes.html and http://www.jaguarpython.com/docs/jaghist.htm.

Even before I knew of the examples above, neither of these counterarguments to the homozygous lethal hypothesis for Spiders held much weight for me. Why? Cause they never disproved the idea of homozygous lethal. It always remained a possibility for me.

But let’s look directly at the predictions of the homozygous lethal hypothesis for Spiders.

Good hypotheses and predictions are based off sound prior knowledge. In this case biological knowledge of the reproductive biology of Python regius.

The following prediction follows logically from the homozygous lethal hypothesis for spider ball pythons and prior biological knowledge.


Prediction: If the homozygous spider genotype (SS) is lethal, then we would expect ~25% chance of slugging in Spider x Spider crosses.


How did I arrive at this prediction? First, we expect ~25% chance of the homozygous genotype from a Spider (Ss) x Spider (Ss) crossing.

Second, let’s look at what would happen to those approximately 1 in 4 chance homozygous spiders if they were embryonic lethals.

In ball pythons, fertilization occurs after ovulation in the infundibulum, an anterior portion of the oviduct .

“Prior to ovulation the growth of the follicles can be reversed and the follicles can be reabsorbed by the ovary. After ovulation there is no mechanism in the body to reabsorb the eggs—once the follicles are ovulated, in one shape or another, something is going to be laid.” See the following link off of kingsnake for more info and the quote. Also special thanks to our own Infundibulum Inspector (xdeus).

So if the homozygous lethal story stands up we would expect that those homozygote that failed to develop due to their genotype would produce nonviable eggs (slugs).

As far as I know there is no known higher rate of slugging from Spider x Spider crosses. If there is no higher rate of slugging, then it seems to me that we must reject the homozygous lethal hypothesis of spiders. If I am wrong or ignorant of some fact, please let me know.

SO what is going on with spiders? Hard to say…..these things take time to work out.

One idea is that perhaps there are homozygous spider adults, but perhaps they are the spiders that fail to reproduce. It could be that the homozygous spiders (SS) don’t have what it takes to produce good eggs and/or sperm. Are such pesky, dud parents more frequent to anyone’s knowledge in spiders?