Synaptonemal complex analysis in spermatocytes of white sturgeon, Acipenser transmontanus Richardson (Pisces, Acipenseridae), a fish with a very high chromosome number

The surface-spreading synaptonemal complex (SC) technique was used to visualize the process of chromosome synapsis in white sturgeon spermatocytes. Pachytene nuclei had various numbers of univalents (0-3) and self-paired foldback elements with no obvious centromeric region (1-7) that may represent accessory chromosomes. The total SC length was 482 ( ±56) µm and the average number of SC elements per nucleus was 139 ( ±3.4). This SC number suggests a higher chromosome number than had been reported in previous mitotic studies (2n = 248 ± 8). There was variation in the SC count both within and between animals. A representative SC karyotype is presented and some differences between the mitotic and meiotic karyotypes for this species are discussed. There was no evidence of multivalent formation, suggesting that the process of diploidization has gone to completion in the males of this ancient polyploid species. Each SC had lateral elements of equal length, and no bivalent exhibited the atypical pairing behavior that is often characteristically associated with heteromorphic sex chromosomes, suggesting that the male is not the heterogametic sex in white sturgeon.