Date of Award

Fall 1973

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)



First Advisor

Rasch, Ellen M.

Second Advisor

Hennen, Sally

Third Advisor

Balsano, J. S.


A method was developed and evaluated for assessing the ploidy of individual fish by cytophotometric measurements of Feulgen-stained scale epithelial nuclei. This technique was used to examine specimens of Amazon molly, Poecilia formosa, its sympatric diploid congener, P.mexicana, putative triploid females associated in nature with P. formosa, and the laboratory-reared progeny of these triploids. The analyses showed that triploid females of the genus Poecilia occur in several different localities of the Rio So to la Marina drainage in northeastern Mexico. When mated to males of P. mexicana in the laboratory, these triploid females consistently gave rise to all-female, triploid young. One instance of triploid offspring produced by a diploid female of P. formosa was found. Alternative mechanisms are proposed to account for the origin and reproduction of triploids. The implications of these models for speciation in the P. formosa- P. mexicana breeding complex are discussed.



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