>>There can never be any male offspring if parthogenesis occurs. All offspring have to be female. There are no male genes present, no Y chromosome.

>>

>>Makes you wonder about something else doesn't it?



Wrong, actually! The real situation is surprising and interesting...



Most lizards and all snakes, like birds which evolved genetic sex-determination independently, do not have an X/Y system like us mammals (where XX is female, XY is male). Instead it's a so-called Z/W system where females, not males, are the heterogametic sex. That is, males are ZZ and females ZW. (In all of these groups there are weird lineages that have lost or duplicated one or other of the sex chromosomes, but we'll ignore those for now).



Genetic (chromosomal) sex determination seems to have evolved several times from environmebntal (usually temperature-dependent) SD as occurs in turtles, crocodilians, tuataras (Sphenodon) and some lizards (some or maybe all Iguania) usually regarded as basal, so that only a single origin of GSD is implied within Squamata. (If BGF and co. are right about 'Toxicofera' there would be at least one reversal to TSD)



The result for would-be parthenogenetic forms with Z/W GSD is that there are two viable routes: (1) by-pass meiosis (the 'reduction division' involved in producing gametes) and duplicate the genome to produce diploid eggs requiring no fertilization (thus an all-female lineage), or (2) allow one of the polar bodies (little sisters of the oocyte produced in meiosis) to re-fuse with it, producing a zygote without intervention of any foreign matter such as spermatozoa.



Varanus komodoensis has taken route (2), with the ability to fertilize its own eggs if no sperm are available, quite different from the well-known all-female parthenogens such as Cnemidophorus spp., Ramphotyphlops braminus, populations of Heteronotia binoei, and various lacertids. That's what makes this a big news story.



What happens when ZW makes gametes and fertilizes itself? You get ZZ, ZW and WW zygotes. ZZ are normal males (OK, a little inbred, but not completely homozygous at every locus). WW just doesn't work: no surviving offspring, apparently. I think I read there's some mechanism preventing daughters from developing, otherwise you'd expect twice as many as sons, but I haven't gone to the trouble of Googling for the articles I read on this just before Xmas (nor had the prior posters, obviously). You may do so now.



Cheers,

John
-----
John D. Scanlon

Riversleigh Fossil Centre

Outback at Isa

Mount Isa, Queensland, Australia

riversleigh@outbackatisa.com.au

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