Giardia sex? Yes, but how and how much?

Although Giardia is of practical importance as a pathogen and has theoretical importance in evolutionary biology, it is not known whether it ever reproduces sexually. Several recent papers have shed light on this problem, without completely solving it. One paper shows that nuclei in the encysted organism can temporarily fuse and exchange genes; this may explain the genetic similarity of the two nuclei in a cell. Another paper demonstrates that Giardia does undergo sexual reproduction with outcrossing. However, we still do not know whether this involves a meiotic or a parasexual cycle, when it occurs, or how common it is. 2009 Elsevier Ltd. All rights reserved.