Sea Squirt Chics Have No Inhibitions

Photo of Botryllus schlosseri from the Swalla Lab website
Yeah thats right they get it on with any male gamete that passes there way. They just give a [rhymes with duck]. Boom chaka-laka-boom. These loose lizzies are all about increasing genetic diversity if you know what I mean. OH, I know you know what I mean!

In the latest Molecular Ecology, Johnson & Yund, explore the promiscuity of tunicates (In order to preserve my somewhat kid-friendly stature, I won't even go into all the possibilities with sea squirts...but feel to in comments!). I like this study, not only for its obvious connotations, but because multiple paternity in a sessile critter is an interesting and fundamental research question. Most studies of promiscuity have focused on mobile fauna. But the uncanny Urochordata, Botryllus schlosseri, has another interesting facet, it alternates between being male and female, with no storage of sperm. Yeah, thats right. Being male is just a phase that Tunicates go through. But this does pose an interesting question.

Johnson & Yund used molecular tools, such as DNA microsatellites and sperm haplotyping to investigate the role of multiple paternity using three measures
1) minimum number of fathers, based on paternal allele frequency from embryos
2) number of sperm haplotypes
3) effective paternity, based on the number of unique sperm haplotypes

They found 15 sperm haplotypes and no effect of local or population level density on any of their measures. I find this interesting because common sense would appear to dictate that colonies would show skewing of sperm haplotypes towards males that are closest to the embryos of the females being sampled as opposed to sperm that has had to travel farther, hence becoming more diluted or fertilizing females closer by, etc. But this doesn't appear to be the case with Botryllus schlosseri from Maine's Damariscotta River estuary.

"The left side of a zooid of Botryllus schlosseri. The center of the system is to the right. Redrawn from van Name (1945). The North and South American ascidians. Bull. American Mus. Nat. Hist. 84:1-476, 31 pls." Taken from Invertebrate Anatomy Online.

So the punchline is that a whopping 90% of broods (using the most conservative measure) have multiple fathers. In fact, the smallest effective paternity from any one population was 2.4, meaning that there at least more than 2 fathers for each brood. Conversely, the highest effective paternity was 14.2! Woowee! Thats a virtual Orgychordata if you ask me! Keep in mind they only genotyped 15-20 embryos out of a potential pool of over 4,000 embryos in a broo.d. So I would definitely say that B. schlosseri is the slut of the invertebrates. But that is not without its potential advantages as outlined in the conclusion by the authors:

"Polyandry to increase genetic diversity is predicted to be favoured only (i) as a mechanism of inbreeding avoidance (ii) under situations of completely unpredictable environmental fluctuation (iii) when there is intense sibling competition, or (iv) when there is some cooperative or compensatory interaction among half-sibs."-Johnson & Yund 2007