Haplobiontic life cycle

12 Pages·2011·0.14 MB·English

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Algae are a diverse group of plants which are known for
exhibiting tremendous diversity in their organization,
structure, habitat, physiology, ecology and reproduction.
They also display morphological and cytological diversity
in their various developmental stages, and this accounts
for diverse life cycles in algae. On the basis of number of
different morphological phases in the life cycle, the algal
life cycles are monomorphic, dimorphic and trimorphic
with one, two and three distinct morphological individuals
in the life cycle, also called as monophasic or
monogenetic, diphasic or digenetic and triphasic or
trigenetic. Based on the occurrence of chromosome
complement or ploidy level of different phases, the life
cycles in algae are haplontic, diplontic, diplobiontic,
haplobiontic and diplobiontic. The haplontic life cycle is
conceived to be primitive, from which diplohaplontic cycle
evolved by a mutation which involved a shift in zygotic
meiosis to mitosis. The diplontic life cycle is ranked as an
advanced type of life cycle.
1. Monogenetic, monomorphic or monophasic life
cycle
This type of life cycle involves only one morphologically
distinct generation which may be either gametophytic or
sporophytic, with the morphological form being either
diploid or haploid. Based on this cytological distinction,
this monogenetic type may be either of haplontic or
diplontic type.
a. Haplontic
This type of life cycle is encountered in majority of
chlorophyceae or green algae such as Chlamydomonas,
Ulothrix, spirogyra, Oedogonium and in some members
of Rhodophyceae such as Bangia. This type of life cycle is
characterized by the dominance of a single generation
which comprises of thalloid vegetative haploid individual
called as gametophyte. The gametophyte reproduces
asexually and vegetatively giving rise to new haploid
gametophyte plants. Thus vegetative and asexual
reproductions don’t have a role in alternation of
generation since they reproduce the same haploid phase.
At the end of the growing season which is marked by the
onset of unfavorable conditions, the gametophyte starts
reproducing sexually. The gametophyte produces
gametes which may be iso- or an-isogametes which on
fusion form the diploid zygote. The zygote is the only
diploid stage in the life cycle and is very brief. It
undergoes meiosis to produce 4 haploid cells or spores
which develop into new gametophytic individuals. Thus
the long dominant haploid gametophytic phase alternates
with a brief zygotic or sporophytic phase; this is not a
true alternation of generation.
This life cycle is characterized by a very long and
dominant haploid gametophyic phase and a brief diploid
zygotic phase with zygotic meiosis.
Diplontic
The diplontic life cycle is predominantly found in
members of Fucales in Phaeophyceae, diatoms of
Bacillarophyceae, Siphonales and Siphonocaldales of
Chlorophyceae. In this type of life cycle, the vegetative
plant is diploid. It is the dominant phase in the life cycle.
It bears sex organs which produce gametes by meiosis.
Thus, this life cycle is characterized by dominance of
diploid phase and gametogenic meiosis which produces
haploid gametes. The gametes on fusion produce the
diploid zygote, which develops into the diploid individual.
Since this life cycle involves the alternation of a long
diploid phase with a brief haploid phase represented by
gametes only, it is called as diplontic life cycle. Like
haplontic life cycle, this life cycle does not exhibit true
alternation of generations since there are no
morphologically distinct sporophytic and gametophytic
generations.
2. Digenetic, dimorphic or diphasic life cycle
This type of life cycle involves two alternating,
morphologically and cytologically distinct individuals of
sporophytic and gametophytic generations with diploid
and haploid chromosome complements, respectively.
Since the two alternating morphological forms or
generations are diploid and haploid, and occur for almost
equal durations during the cycle, this type of life cycle is
called as diplohaplontic life cycle. This type of life cycle is
exhibited by the members of Cladophorales, Ectocarpus
and Dictyota of Phaeophyceae, and Ulva of
Chlorophyceae.
Based on the morphology of the two alternating
sporophytic and gametophytic individuals or generations,
diplohaplontic life cycle is of two types:
a. Isomorphic or homologous diplohaplontic life
cycle
The diplohaplontic life cycle in which alternating diploid
and haploid individuals are morphologically similar or
identical is called as isomorphic or homologous
diplohaplontic life cycle. This type of life cycle is found in
members of Phaeophyceae and Chlrophyceae, such as
Ectocarpus, Dictyota and cladophora, etc. where diploid
sporophyte is identical to haploid gametophyte.
b. Heteromorphic or heterologous diplohaplontic
life cycle
It is that type of diplohaplontic life cycle in which the two
alternating individuals occurring in life cycle are
morphologically very different. In this type of life cycle,
either the gametophyte is more developed or complex in
structure than the sporophyte, or the vice versa. For
example, in Laminariales and Desmarestiales, the
sporophyte is highly developed than gametophyte, while
as in Cutleria the gametophyte is having an elaborate
development than sporophyte.

3. Trigenetic, trimorphic or triphasic Type of life
cycle
This type of life cycle is characterized by the occurrence
of three morphologically different type of individuals
during the life cycle, hence the name trimorphic or
triphasic. This type of life cycle is found in the members
of Rhodophyceae and Phaeophyceae. Here the diploid or
haploid phase may occur twice in the life cycle. If diploid
phase occurs twice, the cycle is called as diplobiontic and
if the haploid phase occurs twice and alternates with
single diploid phase, it is called as haplobiontic.
a. Haplobiontic life cycle
In haplobiontic life cycle, the haploid phase occurs twice
in the life cycle which alternate with a single diploid
zygotic phase. The haploid individuals are the
gametophyte proper and the carposporophyte. This type
of life cycle is found in many members of red algae such
as Betrachospermum and Nemalion. It is also found in
Porphyra and other members of Bangoideae of brown
algae.
In this type of life cycle, the gametophyte bears gametes
which fuse to form zygote. The zygote undergoes meiosis
to form haploid cells, which unusually don’t separate or
behave as spores, but instead undergo repeated mitotic
divisions to form a somatic individual comprising many
haploid filaments. This haploid individual, formed from
initial meiotic and later mitotic divisions of zygote, is
called as carposporophyte.
The haploid carposporophyte is parasitic on the parent
gametophyte, which formed the zygote. Later the
terminal cells of this carposporophyte are transformed
into sporangia, called carposporangia, with each
carposporangium producing a haploid carpospores, which
on liberation develops into the gametophytic plant. Thus
in haplobiontic life cycle zygote is the only diploid phase
in the life cycle which alternates with two haploid phases,
the free living gametophyte and parasitic
carposporophyte.
b. Diplobiontic life cycle
Diplobiontic life cycle is also a triphasic or trimorphic type
of life cycle. Here the diploid phase occurs twice in the
life history which alternate with a single haploid phase.
The gametes formed from the gametophyte fuse to form
the zygote.The zygote formed after fertilization, does not