Meristic co-evolution and genomic co-localization of lateral line scales and vertebrae in Central American cichlid fishes

Meristic traits are often treated as distinct phenotypes that can be used to differentiate and delineate recently diverged species. For instance, the number of lateral line scales and vertebrae, two traits that vary substantially among Neotropical Heroine cichlid species, have been previously suggested to co-evolve. These meristic traits could co-evolve due to shared adaptive, developmental, or genetic factors. If they were found to be genetically or developmentally non-independent, this might require a more general re-evaluation of their role in evolutionary or taxonomic studies. We expanded a previous analysis of correlated evolution of meristic traits (lateral line scales and vertebrae counts) in these fishes to include a range of phylogenetic reconstructions as well as the analyses of 13 Nicaraguan Midas cichlid species (Amphilophus spp.). Additionally, we performed qualitative traits locus (QTL) mapping in a F2 laboratory-reared hybrid population from two ecologically divergent Midas cichlid fish species to discover and evaluate whether genomic co-segregation might explain the observed patterns of meristic co-evolution. Meristic values for these traits were found to morphologically differentiate some species of the Midas cichlid adaptive radiation. Our QTL analysis pinpointed several genomic regions associated with divergence in these traits and highlighted the potential for genomic co-segregation of the lateral line and vertebrae numbers on two chromosomes. Further, our phylogenetic comparative analyses consistently recovered a significant positive evolutionary correlation between the counts of lateral line scale and vertebrae numbers in Neotropical cichlids. Hence, the findings of genomic co-segregation could partially explain the co-evolution of these two meristic traits in these species. Continuing to unravel the genetic architecture governing meristic divergence helps to better understand both trait correlations and the utility of meristic traits in taxonomic diagnoses and how traits in phenotypes might be expected to co-evolve. Los rasgos mer & iacute;sticos son empleados com & uacute;nmente como caracteres fenot & iacute;picos que pueden ser utilizados para diferenciar y delimitar especies recientemente divergentes. Por ejemplo, se ha sugerido anteriormente que el n & uacute;mero de escamas de la l & iacute;nea lateral, as & iacute; como el de v & eacute;rtebras, dos rasgos que var & iacute;an sustancialmente entre las especies de c & iacute;clidos Heroine neotropicales, co-evolucionan. Estos rasgos podr & iacute;an co-evolucionar debido a factores adaptativos, de desarrollo o gen & eacute;ticos compartidos. En el caso de que se descubriera que no son independientes desde el punto de vista gen & eacute;tico o del desarrollo, ser & iacute;a necesario reevaluar su funci & oacute;n en los estudios evolutivos o taxon & oacute;micos. Nosotros hemos ampliado un an & aacute;lisis previo de la evoluci & oacute;n correlacionada de los rasgos mer & iacute;sticos (n & uacute;mero de escamas de la l & iacute;nea lateral y recuento de v & eacute;rtebras) en estos peces para incluir una serie de reconstrucciones filogen & eacute;ticas, as & iacute; como los an & aacute;lisis de las 13 especies de c & iacute;clidos nicarag & uuml;enses Midas (Amphilophus spp.). Adem & aacute;s, realizamos un mapeo QTL en una poblaci & oacute;n h & iacute;brida F2 criada en laboratorio de dos especies de c & iacute;clidos Midas ecol & oacute;gicamente divergentes para descubrir y evaluar si la co-segregaci & oacute;n gen & oacute;mica podr & iacute;a explicar los patrones observados de co-evoluci & oacute;n mer & iacute;stica. Se observ & oacute; que los valores mer & iacute;sticos de estos rasgos diferenciaban morfol & oacute;gicamente algunas especies de la radiaci & oacute;n adaptativa de los c & iacute;clidos Midas. Nuestro an & aacute;lisis QTL identific & oacute; varias regiones gen & oacute;micas asociadas con la divergencia en estos rasgos y destac & oacute; la potencial co-segregaci & oacute;n gen & oacute;mica de la l & iacute;nea lateral y el n & uacute;mero de v & eacute;rtebras en dos cromosomas. Adem & aacute;s, nuestros an & aacute;lisis filogen & eacute;ticos comparativos mostraron una correlaci & oacute;n evolutiva positiva y significativa entre el n & uacute;mero de escamas de la l & iacute;nea lateral y el n & uacute;mero de v & eacute;rtebras en los c & iacute;clidos neotropicales. Por lo tanto, los hallazgos de co-segregaci & oacute;n gen & oacute;mica podr & iacute;an explicar parcialmente la co-evoluci & oacute;n de estos dos rasgos mer & iacute;sticos en los c & iacute;clidos neotropicales. Seguir desentra & ntilde;ando la arquitectura gen & eacute;tica que gobierna la divergencia mer & iacute;stica ayuda a comprender mejor tanto las correlaciones de rasgos como la utilidad de los rasgos mer & iacute;sticos en los diagn & oacute;sticos taxon & oacute;micos y c & oacute;mo podr & iacute;a esperarse que co-evolucionen los rasgos en los fenotipos. We dissect the co-evolution and genetic architecture of lateral line scale counts and vertebrae numbers in Neotropical Heroine cichlids. Our phylogenetic comparative analyses recovered a positive correlation between these two traits. Furthermore, our qualitative traits locus (QTL) analyses identified significant genomic co-localized associations for upper and lower lateral line scales, caudal vertebrae, total lateral line scales, and total vertebrae. Such shared genomic regions could play important roles in cichlid meristic divergence, as they are expected to bias evolutionary divergence.image