Alternative developmental pathways associated with diapause regulated by temperature and maternal influences in embryos of the annual killifish Austrofundulus limnaeus

Abstract

SUMMARYEmbryos of the annual killifish Austrofundulus limnaeus enter a state of developmental arrest termed diapause as part of their normal developmental program. Diapause can occur at two distinct developmental stages in this species, termed diapause II and III. When incubated at 25°C, most embryos enter diapause II, whereas a small percentage of 'escape' embryos develop continuously past diapause II and enter diapause III. Control of entry into diapause II can be altered by maternal influences and the incubation environment experienced by the embryos. Young females produce a higher proportion of escape embryos than do older females. In addition, increasing the incubation temperature from 25 to 30°C induces all embryos to escape from diapause. Surprisingly, escape embryos follow a different morphological and physiological developmental trajectory than do embryos that enter diapause II. Development of anterior structures is advanced compared with that of posterior structures in escape embryos when compared with embryos that will enter diapause II. The difference in timing of development for these two trajectories is consistent with changes observed between two species but is very atypical of variation observed within a species. Importantly, the two developmental pathways diverge early in development, during the segmentation period, when, according to evolutionary theory, constraint on developmental pathways should be relatively high. The possession of alternative developmental pathways in a vertebrate embryo is a novel finding, the ecological and evolutionary importance of which is still unknown, but potentially significant in terms of life-history evolution.

Convergent evolution of alternative developmental trajectories associated with diapause in African and South American killifish

“…In annual species, embryos that entered diapause II became conspicuously different in appearance from direct-developing embryos. This divergence began during mid-embryogenesis, well before diapause was entered, as originally reported for Austrofundulus limnaeus [18]. For example, in N. furzeri direct-developing embryos exhibit a significantly faster rate of head growth and higher heart rate relative to embryos that become committed to entering diapause (trajectory  somite interaction, all p , 0.0001).…”

“…The end result of arrest at one or more of these three stages is that eggs can have a greatly extended development that allows them to traverse the dry season, when adult fish have perished. Embryos, even of the same clutch, routinely follow different developmental trajectories [13,18] which we term the diapause and directdeveloping pathways. Direct-developing eggs skip diapause II and instead undergo continuous development until diapause III is reached, whereas those that enter diapause II exhibit developmental arrest and may not resume development and reach diapause III for a variable length of time, in some cases well over a year.…”

“…Next, we formally test the hypothesis that diapause evolves in response to the colonization of seasonally ephemeral aquatic environments by examining the correlation between diapause and ecological habitat across the killifish phylogeny. We then ask whether diapause is associated with the evolution of alternative developmental pathways [18] by characterizing the embryology of representative annual and non-annual killifish species. Lastly, we probe the adaptive significance of these alternative developmental pathways in the annual species Nothobranchius furzeri.…”

Insulin-like growth factor signaling regulates developmental trajectory associated with diapause in embryos of the annual killifish Austrofundulus limnaeus

“…Embryos that do not enter diapause II develop directly to diapause III, and were considered to 'escape' diapause by Wourms; thus, embryos that do not enter diapause II are termed escape embryos (Wourms, 1972b). Subsequent studies have detailed that entrance into diapause II is an alternative pathway that differs morphologically and physiologically from direct developing escape embryos in all annual killifishes (Furness et al, 2015;Podrabsky et al, 2010). Under laboratory conditions and incubation of embryos at 25°C, young females are known to produce a higher proportion of escape embryos, while older females produce almost exclusively diapausing embryos .…”

Anoxia Tolerance During Vertebrate Development - Insights from Studies on the Annual Killifish Austrofundulus limnaeus

“…In our stock of A. limnaeus, embryos routinely arrest in the laboratory at diapause II and III (Podrabsky and Hand, 1999). Although most embryos arrest in diapause II, some may bypass this stage of diapause and develop directly to diapause III (Wourms, 1972b;Podrabsky et al, 2010a). These embryos are called "escape embryos" (Wourms, 1972b) and very little is known about the physiology of their anoxia tolerance, although they appear to have a different metabolic poise when compared to embryos that enter diapause II (Chennault and Podrabsky, 2010).…”