And a lot depends on the timing. Some crosses have fairly asynchronous
germination, so that when you do the dissection at a time when there is
a good proportion of zygotes that have completed meiosis, a few of them
(those that are a little ahead of the others) have already gone through
the first post-meiotic mitosis, and so are already at the 8-spore
stage. When I start checking them early, and catch the first ones, and
then check every three hours or so after that, I hardly ever see eights
on dissection.
Also, I hardly ever see such asynchronous division from a single zygote
that one spore of the four has divided one extra time, so as to give
five total. . . . although I suppose that might happen with some
crosses. In my crosses, fives msy occur when an unmated cell has stuck
to the wall of a zygote, so it looks like five when you dissect the
zygote. This can be avoided if the dissecting microscope optics are
good enough to allow you to select only the zygotes that look really
round and symmetrical.
Peter Luykx
Dept. of Biology
Univ. of Miami
> Liming Luo wrote:
>>>Now I'm doing Chlamydomonas tetrad analysis. I meet some problems
>>which really confuse me. Sometimes my zygotes produce 8 progeny,
>>sometimes 4. The weird thing is that in some cases, I get 5 progeny
>>from 1 zygote, 3 big and 2 small. It looks like one progeny's
>>already divided into 2. Sometimes I get more than 8 progeny from 1
>>zygote. I don't know how to explain these phenomena.
>>Has anybody met such things before?
>>> Sometimes getting 4 progeny and sometimes 8 is normal, and seems to
> depend on the strains involved, the temperature and light
> intensity, and the age of the zygotes - zygospores that have been
> stored for a couple of weeks usually give a higher proportion of 4's
> rather than 8's.
>> See VanWinkle-Swift (1977) Journal of Phycology 13, 225-231, and The
> Chlamydomonas Sourcebook page 167.
>> I think Forster et al. (1980) Current Genetics 1, 137-153 also
> comment on this phenomenon. In their experiments conditions were
> selected that produced predominantly 8 progeny, so that the
> segregation of chloroplast genes could be followed through the first
> mitotic division.
>> Getting 5 progeny rather than 4 is not so common in my experience,
> but I think your explanation is correct, that one of them has already
> divided again before the zygospore wall is ruptured. If you let all
> five progeny grow up, do the two smaller ones form colonies with
> identical genotypes? If so, then just ignore one of these and score
> the tetrad as if it consisted of four unique cells.
>>> --
> Elizabeth H. Harris
>> Chlamydomonas Center
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