**7.** *Volvox carteri*

*Volvox carteri* is in the Phylum *Chlorophyta*. It is a facultatively sexual species of colonial green algae. The *V. carteri* life cycle can include both a sexual and asexual phase. Under natural conditions, *V. carteri* reproduces asexually in temporary ponds during the spring. However, before the ponds dry up in the summer heat, it becomes sexual forming male and female gametes which can then undergo fertilization to form a desiccation resistant overwintering diploid zygospore. Germination of zygospores involves meiosis and takes place when environmental conditions become favorable, usually the next spring.

*V. carteri* can be induced by heat shock to undergo sexual reproduction [45]. Antioxidants can inhibit this induction, indicating that oxidative stress likely mediates the induction of sexual reproduction by heat shock [46]. Also implicating oxidative stress is the finding that an inhibitor of the mitochondrial electron transport chain, that causes an increase in reactive oxygen species, induces sex in *V. carteri* [47]. Thus the induction of facultative sex, even under natural conditions, may be due to oxidative stress, a condition that causes oxidative DNA damage [47].
