What makes mosses different from other plants

As plants evolved over the past million years, sporophytes have grown more and more dominant. With each new group of land plants: horsetails, ferns, cycads, conifers, and flowering plants, the gameotphyte has steadily become more and more miniscule, until in the flowering plants, it is just a few cells big.

In conifers, the female gameotphyte is a macroscopic lump of tissue inside the cone. If you've eaten a pine nut , you've eaten the haploid female gametophyte of a pine tree, with a diploid embryonic sporophyte embedded inside -- a deliciously oily polyploid sandwich.

In flowering plants -- broadleaf trees, grasses, flowers -- the female gametophyte is only seven cells big, and is hidden deep inside the ovary of a flower. The male is the lowly pollen grain, bane of the allergic, and this is the gametophyte that can make humans suffer. Their size is wildly out of proportion to the misery they inflict. In most cases, they are only two to four cells big. How is it possible for plants -- and for that matter, other algae -- to produce two entirely different organisms from the same bag of genes?

Biologists have been puzzling over this for a long time. The picture is still far from complete, but in a recent paper in Science , scientists from Japan, Australia, and the United States discovered that it is possible to turn the sporophyte developmental program off with the flip of a single genetic switch.

KNOX proteins seem to be involved in sporophyte development. To study their function, they did what biologists almost always do in this situation: they broke them to see what would happen. Biologists call organisms where they break or remove genes to see what stops working "knockout" organisms. The gametophyte moss with the knocked-out KNOX genes looked normal and made functional eggs and sperm, which united to form embryos.

But the development of their sporophyte embryos screeched to a halt about 4 weeks after fertilization. And some of these embryos sported curious filamentous buds. When the scientists cultured these stunted but sprouty sporophytes to see what would happen, the filaments grew into a mass resembling a protonema after a week. A week after that, the protonema produced a leafy bud resembling that made by normal moss gametophytes.

C, a normal "wild type" haploid moss gametophyte sprouting from its filamentous protonema. The diploid wild type moss sprophyte and its normal developmental progression 7 days post-culture. The mutant KNOX knockout sporophyte embryo. At 7 days post culture, it has started to produce suspciously protonema-like filaments in spite of the fact it is diploid.

The mutant sporophyte several weeks later. It has created a protonema from which a gametophyte-like bud has grown. Adapted from Sakakibara et al. But -- and this can't be underlined enough -- this thing that looked like a gametophyte but sprouted from a mutant sporophyte was diploid.

Amazingly, this diploid gametophyte then went on to produce male and female sex organs at the same rate as non-mutant plants. And even more amazingly, it produced functional eggs and sperm, in spite of the fact they too were diploid.

The resulting tetraploid embryos stopped developing at the same stage as the original mutant embryos. When the scientists made single-gene knockouts, it turned out that just one of the two KNOX genes -- mkn6, a transcription factor that flips target suites of genes on or off -- was responsible for the changes observed all by itself. What this implies is that mkn6 represses the haploid genetic program during the diploid generation.

Which means that a single gene can make the difference between two radically different-looking plants in P. That, my friends, is amazing. In the last years, scientists have been discovering that plants seem to have the same sort of puppet-master developmental control genes that animals do, which was big news itself in the decades prior. As early eukaryotes -- nucleated cells, which on Earth is everything but bacteria and archaea -- groped their way toward the first complex, multicellular bodies, they needed some sort of genes to specify This Goes Here, and That Goes There.

In animals, these are called HOX genes , and one of the most incredible discoveries of 20th century biology was that the very same ish base pair signature DNA sequence in these genes -- the famous homeobox -- underlies the embryonic developmental program of everything from worms to fruit flies to humans, specifying what the various body segments will be, whether they be antennae, wings, legs, ribs, vertebrae, or tentacles.

The homeobox of Hox genes codes for a "homeodomain" protein called a transcription factor. This homeodomain transcription factor acts as a DNA binding switch that turns suites of get-stuff-done genes on or off. The same Hox gene that tells a fruit fly embryo how to segment its body tells a developing human where the parts of its brain and spinal column go. Incredibly, they also seem to line up on the chromosome in the same order that they appear in the body, as you can see here taking a look highly recommended!

The really big news here, at least to me, is that plants also have homeobox-containing genes, of which the KNOX transcription factors are one type. The homeobox-containing -- or homeoitic -- proteins seem to have evolved in the earliest eukaryotes and remain important master control genes in nearly all surviving eukaryotes today.

That means genes that controlled your development have recognizable kin in plants, algae, fungi, and protists. Ferns also undergo the alterations of generations.

The sporophyte is prominent over the gametophyte in ferns. The sporophyte is differentiated into true leaves, stem, and root. The leaves of ferns are called fronds. The fronds consist of branched vein systems. That means ferns are vascular plants. The young leaves of the ferns are rolled. The size of ferns can vary from few millimeters to 10 to 25 meters.

Some ferns are terrestrial, and others can be found floating on ponds. The spores of the ferns are produced underside of the fronds. Figure 2: Lifecycle of a Fern. The germination of spores produces a heart-shaped gametophyte with both male and female gametophytes present in the same structure. This allows self-fertilization, which is more successful in dry conditions since the distance to travel by the sperm cells is less.

The fertilization produces a new sporophyte on the gametophyte. The typical life cycle of a fern is shown in figure 2. Mosses: Mosses are small, nonvascular plants that do not have a true root, stem, and leaves and reproduce by the production of spores in stalked capsules. Ferns: Ferns are flowerless, vascular plants with leafy fronds that mainly reproduce by the production of spores.

Mosses: Mosses belong to the phylum Bryophyta. Ferns: Ferns belong to the phylum Pteridophyta. Mosses: Mosses consist of a lot of leaflets. Ferns: Ferns consist of true leaves and stems. Mosses: Mosses contains multicellular rhizoids.

Mosses: Mosses grow up to several centimeters. Mosses: In mosses, the sporophyte depends on the gametophyte. Ferns: In ferns, the gametophyte depends on the sporophyte. Mosses: Mosses consist of male and female gametophytes separately. Fern allies, such as quillworts, horsetails also called scouring rushes and club mosses, are similar to ferns but have enough genetic differences that ecologists have placed them in their own plant divisions. Additionally, bryophytes do not produce flowers and seeds.

Instead, like ferns, they use spores to reproduce. Of the bryophytes, mosses are the most prevalent. Forest Preserve District ecologists and volunteers have identified in DuPage County in a range of locations from dry surfaces, trees and rocks to underwater niches.

Some grow almost anywhere while others only inhabit certain ecosystems. Mosses are typically soft plants that grow in clumps or mats. Like other plants, they produce chlorophyll and undergo photosynthesis, but they do not have true roots.

Because rhizoids are less efficient than roots, mosses generally prefer damp places with low light. When dry, they can go dormant, drawing moisture and nutrients from the green portion of the plant back in the rhizoids, which causes their leaflike structures to curl.

When moist conditions return, they spring back to life, turn green and grow. Their spores grow in beaklike capsules and are usually dispersed by the wind.

Like mosses, liverworts produce spores and live in shady, damp habitats, but they are not as biologically complex as mosses. Though not always the case, a lack of clearly differentiated stemlike and leaflike structures likely means a plant is a liverwort and not a moss. The liver-shaped, leaflike structures on some liverworts give this division of plants its common name.

There are species of liverworts in Illinois, but only about 12 have been seen in Du Page County. The third division of bryophytes, hornworts, is quite rare in Illinois; ecologists have recorded only three species in the state and only one in the Chicago region. Hornworts resemble liverworts but produce their spores on long horn-shaped structures that grow from the base of the plants, among other differences.