Agriculture Reference
In-Depth Information
The juvenile and adult phases of many organisms exemplify hetero-
chrony, and the most dramatic, extreme example of this is metamorpho-
sis. Here, the life cycle includes at least two almost totally different
juvenile and adult life-forms, that if observed separately, would certainly
be classi
ed as different species, for example, a tadpole and a frog or a
caterpillar and a butter
y, almost as different as a snail and an elephant.
Even the model system
Drosophila
exhibits this extreme heterochrony as
a juvenile larva capable of ef
y,
capable of long distance mating. Of course plants provide many exam-
ples of these heterochronic phenotype conversions. This is seen univer-
sally in the pattern of production of axillary shoots (plant architecture)
from the shoot apical meristem and the transition from a vegetative to a
sexually reproductive phase in plants (Weigel and Meyerowitz 1994;
Vollbrecht et al. 2005). Many plants, including
Arabidopsis
, also pro-
duce both juvenile and adult vegetative forms (Willmann and Poethig
2005).
The emphasis by Steven Jay Gould in his topic
Ontogeny and Phylog-
eny
on the idea that those forms result from changing the timing of
ontogeny advanced the use of the term heterochrony. Although Gould
recognized that heterochrony was an important concept in evolution
also, the relationship of ontogeny to evolution remained extremely
controversial until the advent of the Evo-Devo movement. Nonetheless,
the work of Gould and other macroevolutionists (Goldschmidt 1940;
Rensch 1960; Carroll 1997) kept open the idea of the retention of
ontogenic memory in the genome. The Evo-Devo work began to provide
strong evidence for these concepts and allowed them to expand into
modern genetics even though they are so close to Lamarkian views
(Richards 2006; McKinney and McNamara 1991).
cient nutrient acquisition and an adult
B. Ontogenic Programs
The water fern Marsilia exhibits heterochrony as three separate devel-
opmentally consecutive leaf morphologies that recapitulate its apparent
phylogenetic progression from
Regnellidium
and
Pilularia
. Here is a
very good example of the connection between Mendelian genetics, and
Darwinian evolution andWaddington
s epigenetics that almost has been
forgotten by modern molecular genetics. It has been referred to as
“
'
s law of biogenesis
(Gould 1977; Broyles 2009). This concept, which also has been a point
of considerable controversy, teaches that species do evolve by classical
Darwinian natural selection of random mutations. However, it also
suggests that perhaps some of these mutations result in a compressed,
ontogeny recapitulates phylogeny
”
or Haeckel
'
