Biology Reference
In-Depth Information
period (i.e., obligate diapause) in a developmental program. Some evidence
supporting diapause evolution by genetic accommodationmight be provided
by “nondiapausing” strains of some lepidopteran species (e.g.
Lymantria
or
Bombyx
strains) in which diapause can be elicited by mis-regulating a specific
hormone (see above).
Mechanisms underlying the evolution of diapause trajectory might also
enhance the environmental sensitivity of other developmental modules and
promote the evolution of multiple polyphenic, diapause-linked traits (such
as those found for
B. mori
and
Araschnia levana
). Indeed, sensitizing changes
altering the timing, or the levels, of developmental hormones during dia-
pause evolution might shift, or sensitize, the hormonally sensitive period
of a diapause-linked trait, promoting genetic accommodation. These altered
hormonal dynamics might facilitate the evolution of different life cycles, as
discussed for other issues (
Heyland et al., 2005; Pfennig et al., 2010
).
8. THE KILLIFISH DIAPAUSING EMBRYO:
A DEVELOPMENTAL ANALOGY IN A VERTEBRATE
Diapause is also induced in some vertebrate embryos without causing
any developmental aberration, analogous to insect diapause. An example is
the annual killifish,
Austrofundulus limanaeus
(Cyprinodontiformes; Apo-
cheiloidei). Although its diapause is not a hibernanal dormancy in response
to an upcoming winter but, is instead a “hypoxia-induced” diapause, the
developmentalmechanismis, nevertheless, reminiscent of what occurs in lep-
idopteran species. Killifish live in ephemeral ponds that seasonally undergo
fast desiccation during the dry season. Hence, survival of the species depends
entirely on the buried (dormant) embryos that develop during the rainy
period, once the ponds are refilled by seasonal rain (
Berois, Arezo, Papa, &
Clivio, 2012; Podrabsky, Carpenter, & Hand, 2001, Podrabsky & Hand,
1999; Podrabsky, Lopez, Fan, Higashi, & Somero, 2007
). Killifish embryos
block developmental growth and metabolism (
80-90% reduction) by
triggering diapause potentially at three different stages: as an early embryo
(diapause I, rare), as 38-somite embryos (diapause II, the embryo has devel-
oping endoderm-derived organs, a beating heart, circulating red blood cells,
optic cups and a maturating nervous system), or as “hatch-ready” embryo
(diapause III, obligate, fully developed embryo waiting the appropriate cues
to hatch) (
Berois et al., 2012; Podrabsky, Garrett, & Kohl, 2010
). Diapause
trajectory is strictly associated with modifications of the timing of develop-
mental growth (
Fig. 8.6
)(
Podrabsky et al., 2010
). Diapausing embryos are
