Last year, Nicholas Holland and colleagues examined an interesting case of hybridization between lineages separated for more than 100 million years. Lancelets are primitive chordates with a notochord instead of vertebrae and the present diversity dates to the Mesozoic era, yet the authors found that fertilization and development commenced between them in the laboratory.
Ultimately, one would like to move from analyzing the influence of hybridization on developmental mechanisms at the level of a few key genes to examining the problem in the context of the overall dynamics of gene networks and posttranslational phenomena. As more is learned about these aspects of development, it should be especially interesting to study how hybrid morphology is influenced by the coordinated functioning of markedly divergent parental genomes. Currently, in terms of parental divergence times, the genomes of Branchiostoma and Asymmetron are the most divergent yet mixed to produce unequivocal hybrid offspring surviving at least to the phylotypic stage—in this instance to the chordate pharyngula.
The hybrid embryos developed only to the larval stage, and the authors did not know whether nutrition might have allowed further development. But it is a very interesting case of the long-term conservatism of early development in a basal chordate.
Holland ND, Holland LZ, Heimberg A. 2015. Hybrids Between the Florida Amphioxus (Branchiostoma floridae) and the Bahamas Lancelet (Asymmetron lucayanum): Developmental Morphology and Chromosome Counts. The Biological Bulletin 228:13-24.