From Andy Wang to bionet.chlamydomonas:
I have been doing a lot of crosses lately and have found a way to determine the sex of tetrad products. The products can either be grown on regular plates or on N- plates.
Sterile water or N- medium (about 100 ul) are placed in the wells of a microplate (with flat bottoms) and tetrad products can be transferred into the wells with toothpicks. I have kept them this way (covered) for more than 24 hrs and still get good mating activity. Tester stocks of either mating type can then be pipeted into the wells. I usually left them over night. The next morning, the plates are examined under the dissecting scope. Positive mating will produce a pellicle at the bottom of the well which, if not sure, can be examined with the help of a toothpick. One advantage of this method is the small number of cells needed. The other is having two mating pairs in adjacent wells for comparison. If one has an inverted microscope, activity in the wells can also be observed. I hope this will be of some help to someone.