Frimousse – a spontaneous ascidian mutant with anterior ectodermal fate transformation

Abstract

Among vertebrates, the cellular and molecular mechanisms of neural induction appear largely divergent [1], thus providing little indication of the ancestral chordate mode. Ascidians, which belong to a sub-phylum of the chordates (Urochordata) long separated from the lineage leading to the vertebrates, may provide insight to these questions [2,3]. We have investigated neural specification in ascidians using a genetic approach. Ascidians are hermaphrodites, and the ability of ascidians of the genus Ciona to self-fertilize provides a rapid means for identifying recessive zygotic mutations [4,5]. We screened Ciona intestinalis for naturally occurring mutations, which can be detected in a large percentage of the wild population One such mutant is frimousse (frm). In the ascidian larva, the sensory vesicle, the putative homolog of the vertebrate foreand midbrain [6], is the most prominent feature of the anterior CNS, and contains two pigmented sensory organs, the ocellus and the otolith, which are sensitive to light and gravity, respectively (Figure 1A). The frimousse (frm) mutant was initially identified in larvae of a self-fertilized brood due to the absence of the pigmented sensory structures in ~25% of individuals (Figure 1B). The founder was subsequently crossed to wild-type individuals, and the mutant line has been maintained for over six generations without a change in phenotype. Conservationists have good reason to ring alarm bells about the decline in numbers of many species worldwide. Most are linked to the increasing intrusion from human activity, but one decline has proved a major puzzle: all around the world there appears to be a decline in numbers of amphibians and often in habitats with minimal human influence. A new study has looked at the falling numbers of the mountain yellow-legged frog (Rana muscosa) in the Sierra Nevada in California. Reporting in the Proceedings of the National Academy of Sciences (published online) a researcher has for once got some good news. Not only does he determine one major factor in the frog population's decline, he describes how to reverse it. The key factor was thought by Vance Vredenburg, at the University of California, Berkeley, to be the widespread introduction of both the rainbow trout and brook trout into lakes in these mountains where the frogs live. His study experimentally manipulated the presence and absence of these two fish species. From 1996 to 2003, the introduced trout were removed from five lakes in a remote protected area, and 16 nearby lakes were used as controls, eight with introduced trout and eight without. To determine the vulnerable life stages, rainbow trout were placed in cages in three lakes containing amphibians. The study found that removal of introduced trout resulted in rapid recovery of frog populations and in the caging experiment, tadpoles were found to be vulnerable to trout predation. The results suggest that the widespread introduction of these fish has had a major impact on frog populations in this region of California, but that removal of them can lead to a rapid and dramatic reversal in the amphibians' fortunes. Other observations in Costa Rica, Spain and field experiments in Australia implicate introduced trout in the decline of local frog populations, but this study has shown for this frog species what has actually been happening and what can be done. Trout out