The liverwort genus Sphaerocarpos was the first genus in which sex chromosomes were known in plants. Many bryophytes possess sex chromosomes (X & Y chromosomes, or designated U & V to refer to their haploid condition) which may play a role in gender determination. Bryophytes can be monoicous (bisexual) or dioicous (unisexual). Gametangia in monoicous bryophytes can be autoicous (♂ & ♀ gametangia in separate clusters), paroicous (♂ & ♀ gametangia in separate groupings but one cluster), or synoicous (♂ & ♀ gametangia intermixed in same cluster). Monoicy frequently has arisen through hybridization and polyploidy. Transitions from monoicy to dioicy and vice versa have happened multiple times. There have been more changes from monoicy to dioicy than the reverse in mosses, whereas the opposite was the case in hornworts. McDaniel et al. suggested that dioicy works best when there are advantages to both sexes for being separate. Sperm dispersal begins with bursting of the antheridium, often accompanied by movement with surface tension of water drops. In thallose liverworts, sperm are often expelled explosively into the air. Sperm dispersal is usually accomplished by movement through a water film or by splashing and is sometimes aided by gravity. But some species have their sperm dispersed by invertebrates, including insects and mites. Dispersing sperm are known to survive as much as 200 hours and travel distance is known up to 230 cm. Travel distance and weather seem to be the most important factors in determining the success of fertilization in bryophytes