Heterogamety in Teleostean Fishes

Teleostean fishes constitute a proliferous evolutionary grade, which shows a correspondingly diverse array of sex‐mechanisms. Although most species are gonochorists, many groups have independently evolved monoecy as a response to varying degrees of breeding isolation. On the other hand, cytologically expressed heterogamety distinguishes several groups, especially those represented by the deep‐sea species investigated, which belong to primitive and intermediate orders. Acetic‐orcein squashes of kidney, spleen, and gonad reveal heteromorphic chromosome pairs in males of nearly half the 25 species of deep‐sea teleosts examined. Within one family, the Y chromosome varies interspecifically in size from the second largest to the smallest element in the complement. Most such pairs are identifiable as atypically behaving bivalents associated end‐to‐end; some of these, perhaps, are expressed as heterochromatic bodies in preleptotene. The dimorphic components segregate in the secondary spermatocytes. Two karyotypes contain a large, unpaired “X” chromosome, suggesting an XO mechanism. Proportionately less heterogamety is detectable among shallow‐water teleosts, mainly in predominately freshwater groups. Two species are presumably heterogametic in the female. More common than previously thought, chromosomal heteromorphy apparently has arisen independently in diverse teleostean groups. Possible evolutionary stages are exemplified within two unrelated genera, one of deep‐sea salmoniform fishes, the other of estuarine and freshwater cyprinodontoids: one member of a heteromorphic chromosome pair, which may originate as a pericentric inversion, sustains subsequent genetic loss and becomes noticeably shorter than the other. Generally, the larger element is one of the largest chromosomes in the complement, while the smaller element varies in size from smallest to subequal the X.