Cross-breeding studies in seven Artemia franciscana strains from Mexico JORGE CASTRO MEJIA1, THALIA CASTRO BARRERA1, JOSE LUIS ARREDONDO FIGUEROA2, GERMAN CASTRO MEJIA1, ANDRADE RAMON DE LARA1 and AIDA MALPICA SANCHEZ1 1 Universidad Aut½noma Metropolitana-Xochimilco. Depto. El Hombre y su Ambiente. Laboratorio: Producci½n de Alimento Vivo Calz. Del Hueso No. 1100. Colonia Villa Quietud. Mexico, 04960, D.F. 2 Universidad Aut½noma Metropolitana-Iztapalapa. Planta Experimental de Producci½n Acu£cola.San Rafael Atlixco No. 186. Colonia Vicentina. Mexico, 09340, D.F.   Abstract Cross-breeding experiments were performed on seven Mexican strains of Artemia franciscana to determine their status of reproductive isolation. The populations used in this study were: Yavaros (YAV), Ohuira (OHUI), Juchitan (JUCH), Real de las Salinas (R.SAL), Cuatro Cienegas (C.CIEN), Las Salinas de Hidalgo, San Luis Potosi (SLP) and Texcoco (TEX). Each population was cultured in a 40-l tank. Feeding consisted of a mixture of Tetraselmis sp. and Dunaliella sp. When the organisms had reached the pre-adult stage, males and females were separated. For cross-breeding experiments, one female from one population and two males from another population were placed in a 250-ml flask; the same procedure was followed for the reciprocal crosses. Once the presence of nauplii (F1) was observed, they were placed in another flask until they were sexually mature. From the 42 cross-breeding tests performed, only seven presented 100% reproductive success, while 15 were not successful. For F2, the number of successful cross-breedings dropped drastically, showing increased sterility. Only the crossbreeding involving female TEX/males R.SAL maintained high nauplii production in F1 and F2 (445 and 401, respectively). We can conclude that the studied Mexican populations of Artemia franciscana are in the process of building post-zygotic reproductive isolation (hybrid breakdown), caused by the ecological preferences and/or adaptation of each population to its specific habitat. This might be reflected in their genotype, inducing pre- and post-mating alterations.   < back .