Sex ratios.

skinheaddave

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I realized today how far behind my scientific understanding of tarantulas is to my scientific understanding of scorpions. I had what should have been a very basic question and didn't know the answer -- hence my posting here.

How is sex determined in tarantulas? Is it temperature-dependent? Is it geneticaly based? On a practical scale, if I buy a lot of sibling s'lings are they likely to all be one sex or should I get a mix? If there is a mix, is the ratio 50:50 or something else? What if they have had time to canabalize each other? Same ratio, or do males tend to lose out?

I ask for two reasons. The first is that my girlfriend and I have inherited a breeding project as part of a collection we bought from a guy who was getting out of spiders. We have a mature female Brachypelma vagans and 15 juvies who are apparently CB siblings. What are the odds one will be male?

Also, I am thinking of buying 10 Usumbara slings to eventualy mate with one of three females I currently own. Is this a wise move (not counting that it is going to cost money that might be better spent on more scorpions ;) )?

Cheers,
Dave
 

King_Looey

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As far as i know, their gender is determind randomly, otherwise people would mostly breed females, as they are more expensive.
 

MrDeranged

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There was talk for a while that there might be some way to control breeding ratio's with temperature, but I don't think they came to anything. From what I know, sacs are pretty much equal opportunity in the beginning. The problem is that males grow quicker than females so that if you allow cannibalization, there is a greater chance of your females being eaten. Out of your 15 juvies, I'd say theres pretty much at least a 100% chance that at least one will be male. If you buy a lot of siblings, you should get a mix, depending on how many you buy and what amount, if any, of cannibalization did occur. Basically with slings, it's a crap shoot.

Scott
 

Code Monkey

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To the best of my knowledge, Ts use a chromosomally determined gender just like us and most organisms. Someone published a temperature study on gender with a clutch of Usumbara slings in the ATS journal and the conclusion was that temp had zero effect, you wound up with about 50/50 regardless.
 

Code Monkey

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Definitive answer time (got my copy of Biology of Spiders yesterday):

Spiders do use a chromosomally determined sex ratio, albeit a weird one. They don't have the X & Y system most of you should be familiar with but rather a funky X chromosome system. Females have 4X chromosomes (XXXX), males have 2X chromosomes (XX). So a male sperm has 0X chromosomes and a female sperm has 2X chromosomes; eggs all have 2X.
 

skinheaddave

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Excellent. I like definitive answers. That system would definitely put the ratio at 50:50. Did the book, per chance, discuss the evolution of this particular system? I'm a little rusty on my invertebrate genetics, but if I remember correctly, a system using only different numbers of a single chromosome is quite common in inverts. I wonder how common these large numbers are, and hence whether it evolved from an X/XX system?

Cheers,
Dave
 
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