heterochrony An evolutionary change in develop-
mental timing in the relative time or rate of appearance
ordevelopment of a character. The morphological out-
comes of changes in rates and timing of development
are paedomorphosis (less growth) or peramorphosis
(more growth). Peramorphosis is the extended or exag-
gerated shape of the adult descendant relative to the
adult ancestor; its later ontogenetic stages retain char-
acteristics from earlier stages of an ancestor. Paedomor-
phosis is where the adult descendant retains a more
juvenile looking shape or looks more like the juvenile
form of its adult ancestor; its development goes further
than the ancestor and produces exaggerated adult
traits. Paedomorphosis can happen as a result of begin-
ning late (postdisplacement), ending early (progensis),
or slowing in the growth rate (neoteny). Likewise, per-
amorphosis can result from starting early (predisplace-
ment), ending late (hypermorphosis), or having a
greater growth rate (acceleration).
heterocyst A large, thick-walled, specialized cell
working in anoxic (oxygen absent) conditions that
engages in nitrogen fixation from the air on some fila-
mentous cyanobacteria; an autotrophic organism.
heteroecious A parasite that starts its life cycle on one
organism and then affects a second host species to com-
plete the cycle, e.g., peach-potato aphid (Myzus persicae).
heterogamy Producing gametes of two different
types from unlike individuals, e.g., egg and sperm. The
tendency for unlike types to mate with unlike types.
heterolysis (heterolytic cleavage or heterolytic fission)
The cleavage of a bond so that both bonding electrons
remain with one of the two fragments between which
the bond is broken.
heteromorphic Having different forms at different
periods of the life cycle, as in stages of insect metamor-
phosis and the life cycle of modern plants, where the
sporophyte and gametophyte generations have different
morphology.
Heteroptera Asuborder known as true bugs. They
have very distinctive front wings, called hemelytra. The
basal half is leathery and the apical half is membranous.
They have elongate, piercing-sucking mouthparts.
Worldwide in distribution, there are more than 50,000
species. Two families are ectoparasites. The Cimicidae
(bed bugs) live on birds and mammals including
humans, and the Polyctenidae (bat bugs) live on bats.heteroreceptor ARECEPTOR regulating the synthesis
and/or the release of mediators other than its own ligand.
See alsoAUTORECEPTOR.heterosexual Having an affection for members of
the opposite sex, i.e., male attracted to female.
See alsoHOMOSEXUAL.heterosis Vigorous, productive hybrids that result from
a directed cross between two pure-breeding plant lines.heterosporous Producing two types of spores differ-
ing in size and sex. Plant sporophytes that produce two
kinds of spores that develop into either male or female
gametophytes.heterotrophic organisms Organisms that are not
able to synthesize cell components from carbon dioxide
as a sole carbon source. Heterotrophic organisms use
preformed oxidizable organic SUBSTRATEs such as glu-
cose as carbon and energy sources, while energy is
gained through chemical processes (chemoheterotro-
phy) or through light sources (photoheterotrophy).heterozygote A diploid organism or cell that has
inherited different alleles, at a particular locus, from each
parent (i.e., Aa individual); a form of polymorphism.heterozygote advantage(overdominance) The evo-
lutionary mechanism that ensures that eukaryotic het-
erozygote individuals (Aa) leave more offspring than
homozygote (AAor aa)individuals, thereby preserving162 heterochrony