A sporophyte () is one of the two alternating multicellular phases in the life cycles of and algae. It's is a diploid multicellular organism which produces asexual Spore. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the
zygote produced when a
haploid egg cell is fertilized by a haploid
sperm and each sporophyte cell therefore has a double set of
, one set from each parent. All
Embryophyta, and most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploid
gametophyte phase. In the
Spermatophyte, the largest groups of which are the
(bare seeds) and
angiosperms (fruiting plants), the sporophyte phase is more prominent than the gametophyte, and is the familiar green plant with its roots, stem, leaves and cones or flowers. In flowering plants, the gametophytes are very reduced in size, and are represented by the germinated
pollen and the embryo sac.
The sporophyte produces (hence the name) by meiosis, a process also known as "reduction division" that reduces the number of chromosomes in each spore mother cell by half. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of . Meiosis in the diploid sporophyte provides a direct DNA repair capability for dealing with DNA damages, including oxidative DNA damages, in germline reproductive tissues.
The mature gametophyte produces male or female (or both) by mitosis. The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte. This cycle is known as alternation of generations or alternation of phases.
Examples
Bryophytes (
,
Marchantiophyta and
hornworts) have a dominant gametophyte phase on which the adult sporophyte is dependent for nutrition. The
embryo sporophyte develops by
cell division of the zygote within the female sex organ or
archegonium, and in its early development is therefore nurtured by the gametophyte.
Because this embryo-nurturing feature of the life cycle is common to all land plants they are known collectively as the
embryophytes.
Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar (isomorphic). An independent sporophyte is the dominant form in all , , , gymnosperms, and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores (isosporous or homosporous) but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female tending to be larger, and fewer in number, than the male .
Evolutionary history
During the
Devonian period several plant groups independently evolved
heterospory and subsequently the habit of endospory, in which the gametophytes develop in miniaturized form inside the spore wall. By contrast in exosporous plants, including modern ferns, the gametophytes break the spore wall open on germination and develop outside it. The
of endosporic plants such as the seed ferns developed within the sporangia of the parent sporophyte, producing a miniature multicellular female gametophyte complete with female sex organs, or archegonia. The
oocytes were fertilized in the archegonia by free-swimming
sperm produced by windborne miniaturized male gametophytes in the form of pre-pollen. The resulting zygote developed into the next sporophyte generation while still retained within the pre-
ovule, the single large female meiospore or megaspore contained in the modified
sporangium or
nucellus of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of
of the kind produced by gymnosperms and angiosperms today. The rRNA genes seems to escape global methylation machinery in bryophytes, unlike seed plants.
See also
-
Alternation of generations
Further reading
