Many reptiles have no sex chromosomes. Instead, their gender is determined by temperature. In crocodiles, for example, males are hot: eggs incubated in sand above a certain "pivotal temperature" almost always hatch males. That could spell trouble, because Earth is warming so fast that natural selection may not have time to adjust pivotal temperatures. Female crocodiles may become scarce. Some experts think fish are in the same hot water because temperature-dependent sex determination, or TSD, occurs in many species. But a critical analysis of the fish literature by Natalia Ospina-Alvarez and Francesc Piferrer, both of the Marine Science Institute in Barcelona, Spain, casts doubt on that assertion. The pair point out that fourteen of the twenty diverse fish genera previously reported as having TSD in fact have sex chromosomes, and that it takes unnatural temperatures foundrnonly in the laboratory to masculinize their genetic females.
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