IS A BRYOPHYTE SPOROPHYTE A PARASITE



IS A BRYOPHYTE SPOROPHYTE A PARASITE?

I was reading Chapter 2-1 of your book on Bryophyte Ecology; at the end, the 'Summary' states that the gametophyte supports a parasitic sprophyte. I believe this is a mistake that has caused long discussions among teachers in our introductory courses. The growing sporophyte causes no damage to the gametophyte and once in the open, it is photosynthetic. Clearly, there is a misconception of the relationship between the gametophyte and the sporophyte. To me, it would be like saying that my fingers or hands are parasitic on my trunk! What do you think?

Claudio Delgadillo

  In reading my Chapter 2-1 Summary, Claudio Delgadillo has raised the question of whether we can truly consider the sporophyte of a bryophyte to be a parasite on the gametophyte.  He argues that it would be like considering my hand to be a parasite on my body.  He further argues that the sporophyte causes no damage to the gametophyte and that it is photosynthetic for part of its existence. Claudio's points are well founded, so I decided to explore it a bit on the internet to see how others consider the relationship.

  The first definition I found for parasite is:  an animal or plant that lives in or on a host (another animal or plant); it obtains nourishment from the host without benefiting or killing the host.  I agree that the sporophyte is capable of fixing photosynthate, but it also derives nourishment from the gametophyte, so I feel that part of the definition is satisfied.  Thus, the question becomes, is the sporophyte a different plant by virtue of being a different generation? To follow the human analogy, is a fetus a parasite on its mother?

  I would consider that the sporophyte does do the gametophyte harm, as discussed in literature cited in Chapter 5-9 of my bryophyte ecology book .  Its requirement for energy from the gametophyte greatly reduces the biomass gain of the gametophyte compared to those gametophytes not supporting a sporophyte, at least in the species studied.

  At this point, it would seem one could argue either way.  Convention becomes our guide, as language is a product of human use.  The University of British Columbia web site on bryophytes states "The sporophyte grows partly as a parasite on the gametophyte."   The University of Illinois Chicago website states "Sporophyte is a parasite."  The World of Biology Website states "In Seed Plants, the Gametophyte is a very small Parasite of the Sporophyte."  "The young sporophyte, as for ferns, develops initially as a parasite on the gametophyte."  Similarly, in Brittanica, "Among the gymnosperms the male gametophyte is much reduced and is a parasite on the sporophyte for only a short time."  The Berkeley Tree of Life website refers to algae:  "Small sporophyte is dependent on a larger dominant gametophyte."  Clearly the concept of a plant being parasitic on its other generation is not unique to bryophytes.

  On the other hand, a German website, referring to bryophytes, states "If these both wouldn't belong to the same species, the sporophyte could be called a parasite on the gametophyte."

  This brings us full circle.  Can we legitimately call a species a parasite on itself?  It seems to me we have broadened our use of the term to include such instances.  We have a history of doing this, particularly in ecology, where usage of the term plant strategy became very controversial, but it has stuck in the literature.

Janice Glime

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

RESPONSES

I would vote no - the sporophyte is not a parasite on the gametophyte.

Why no- because it is part of the life cycle of the system, and the gametophyte clearly benefits from the interaction (offspring production) . Although I suspect that the connected gametophytic tissue dies in the process.  Clearly we can  ask what are the true effects of the interactions to both players (I am ignoring the male/sperm and spores) I suspect we will need to carefully argue some of the possible +, - or 0 effects.

Using parasite sounds provocative and draws  interest from others (etc the public/students).   To think a fetus / seed is a parasite on the parent is another way to look at parent/offspring interactions and can even be very useful in tapping into theory from another field to understand this relationship/interaction.

Another interesting case are the males of some deep sea fish that are very small and are parasitic on females :) .   (these are clearly different individuals)

I guess diversity does interesting things to definitions

take care

Nicholas

-- 

D. Nicholas Mc Letchie

Ph 859 257 6786

fax 859 257 1717



Department of Biology

101 Morgan Building

University of Kentucky

Lexington, KY 40506

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Janice,

I say yes.  It is definitely not the same as saying my hand is parasitic on the rest of my body.  I do not believe that there is a similar situation in animals for a comparison.

Sincerely,

Keith Bowman

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

No.

If we say yes, I think that we're abusing the word "parasite''. I agree with

what the German website says, since both the gametophyte and sporophyte belong

to the same species, we can't call the sporophyte a parasite.

As to the definition "parasite-" it obtains nourishment from the host

without benefiting or killing the host'', I'll say that the sporophyte does

benefit the host. Without a successful reproduction system, there will be no

more gametophytes. That's how life is.

Hua

--

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

This is a very interesting question which brought back many memories of my first academic job interview at the University of Florida. My interview seminar dealt with carbohydrate metabolism in source and sink tissues. While the main part of my talk dealt with carbohydrate metabolism in potatoes, I had recently done work on carbohydrate metabolism in the gametophyte and sporophyte of several moss species.  I was asked the question, "If mosses lack vascular tissue how could you talk about a source and a sink." I know this is a little off your question but it got me to thinking about the relationship between the sporophyte and gametophyte. I don't think I would class the generations as being parasitic since each generation derives a benefit from the other. I think the association would be more mutualistic than parasitic. The sporophyte generation would get the more immediate benefit from the photosynthate from the gametophyte but, the moss as a whole would benefit due t! o genetic

Cyndy Galloway

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

No. How about "matrotrophophyte" (noun) or "matrotrophophytic" (adj.)

from matrotrophy. Matrotophy has common usage in zoology and botany.

Why give the sporophyte the negative connotation as being a parasite?

I see no value in this other than to convey the concept of partial

dependence on the gametophyte. It is not a parasite anymore than a

human fetus as you point out.

Thank you for the opportunity to offer an opinion.

Paul G. Davison

UNA Box 5232

Department of Biology

University of North Alabama

Florence, AL 35632

Phone (256) 765-4434

Fax (256) 765-4430



++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

From my point of view one generation of a species can not be the

parasite of the other generation.

Without the sporophyte there would be not sexual reproduction of mosses.

As a consequence, this would harm the evolution of mosses.

So, there are two more biological arguments against this concept:

1. Mosses benefit from the alteration of generations in terms of

evolution and dispersal.

2. Parasitism is an option (a thread) to the host without benefit for

the host. Alteration of generations is far more than an option.

Warm regards,

Ralf.

--

Ralf Reski

Professor

Dean Faculty of Biology

Head Plant Biotechnology

University of Freiburg

Schaenzlestrasse 1

D-79104 Freiburg

Germany

Fon: +49 761 203-6969

Fax: +49 761 203-6967

ralf.reski@biologie.uni-freiburg.de



+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Yes, but I'm open to alternate language and look forward to the responses

you get! A fetus hormonally mines its mother of calcium and nitrogen, so to

me it is a very well-adapted parasiste...even using specialized globins for

oxygen transfer.

FWIW: I never use "fertilization" when I mean "syngamy" because of the

obvious inaccuracy and confusing interpretation of the former...for

botanists "fertilization" often means application of minerals, compost, etc.

rather than union of gametes...and females are fertile whether male gametes

are available or not...so just as NASA forced physiologists to stop using

geotropism when they mean gravitropism...we do need to make sure our terms

accurate.

An interesting thread!

ross

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

They are separate organisms, despite their genetic relationship.

-- 

__________________________________

Martha E. Cook

Associate Professor of Botany

Illinois State University

Department of Biological Sciences

Campus Box 4120

Normal, IL 61790-4120  USA

Phone: 309-438-8549

Fax:  309-438-3722

E-mail: mecook1@ilstu.edu



+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Parasitism is a type of relationship that involves two individuals of different species (from Wikipedia: “two organisms which are phylogenetically unrelated”). Thus, I think it is not well used to describe the relationship between gametophyte and sporophyte in bryophytes, because in this case there is one only species (and maybe one only individual, or how many individuals are there in a fructified gametophore?). As an exception, parasitism can occur within the same species, as in the case of brood parasitism: brood parasites lay their eggs in the nests of hosts of the same or different species. Intraspecific brood parasitism has been reported for a range of insect, bird and fish species.

In addition, in parasitism one individual (the parasit) benefits and the other (the host) is harmed. In bryophytes, gametophyte growth may be negatively affected by sporophyte production, but spore formation may contribute to genetic diversity and thus to environmental adaptability and ecological success. Thus, it is debatable if the supposed “host” (gametophyte) is harmed or benefited or both.

I think that the term “parasitism” is being successful because of a simple question of fashion, in the line of similar “sensationalist” terms such as “plant strategies” or “invasive alien plants”.

There is a term (auxotrophy) that sometimes has been used to describe the relationship between gametophyte and sporophyte in bryophytes. From Wikipedia:

“Auxotrophy is the inability of an organism to synthesize a particular organic compound required for its growth (as defined by IUPAC). An auxotroph is an organism that displays this characteristic; auxotrophic is the corresponding adjective. Auxotrophy is the opposite of prototrophy.

In genetics, a strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesize an essential compound. For example a yeast mutant in which a gene of the uracil synthesis pathway is inactivated is a uracil auxotroph. Such a strain is unable to synthesize uracil and will only be able to grow if uracil can be taken up from the environment. This is the opposite of a uracil prototroph, or in this case a wild-type strain, which can still grow in the absence of uracil. Auxotrophic genetic markers are often used in molecular genetics.

Researchers have used strains of E. coli auxotrophic for specific amino acids to introduce non-natural amino acid analogues into proteins. For instance cells auxotrophic for the amino acid phenylalanine can be grown in media supplemented with an analogue such as para-azido phenylalanine. Aminoacyl tRNA synthetases recogonize the analogue and catalyze its binding to a tRNA which subsequently transfers the amino acid (non natural in this case) to a growing polypeptide chain during protein translation.

It is important to remember that many living things, including humans, are auxotrophic for large classes of compounds required for growth and must obtain these compounds through diet (see vitamin, essential amino acid, essential fatty acid).”

The term “auxotrophy” does not fit exactly with the relationship between gametophyte and sporophyte in bryophytes, but it may help. Probably this relationship is so unusual in nature that we need to invent a new term.

With best regards,

Javier

-- 

**********************************

Dr. Javier Martinez-Abaigar

Universidad de La Rioja

Complejo Cientifico-Tecnologico

Avda. Madre de Dios 51

26006 Logroño (La Rioja)

(España, Spain)

Tel.: 34+941+299754

Fax: 34+941+299721

javier.martinez@unirioja.es



*********************************

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Yes, I consider the sporophyte parasitic on the gametophyte. Matritrophy is key

to sporophyte survival, but the word is too loaded. The sporophyte is

dependent, like a child is on its mother, for as a child can breath so too may

a sporophyte photosythesise (perhaps even using atmospheric carbon via its

stomata, in the case of mosses).

C. C. C. Chater, UK

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

This brings to mind that I once read something about excised hornwort

sporophytes growing in vitro--can't remember details or the source. I once

found a population of hornworts where rain runoff had completely covered the

gametophytes, but the sporophytes (many of them several cm in length)

appeared quite healthy and growing vigorously. There were many dozens of

them, growing densely enough that the population resembled a patch of

strange graminoids. The gametophytes I retrieved from the sandy soil were

black and shriveled so far as I could see with a handlens. I did not examine

any of them in the laboratory, but it did seem that the gametophytes were

serving as little more than perhaps a sort of wick between sporophyte and

soil solution.

I am wondering if there have been any observations of further maturation in

the relatively large chlorophyllose young sporophytes of certain mosses if

they are excised and maintained in vitro?

Dan Marsh

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

About the question, I think that the sporophyte is not parasite of gametophyte, first is the same species although a different generation. The development of sporophyte is necessary to complete the life cycle, so if the sporophyte complete its life and produce a lot of spores it will be  good for the species ( including gametophyte).

Is the root of a tree a parasite of the photosynthetic part of the tree? or Is the female gametophyte of angiosperms parasitic of sporophyte ? I think no.

sincerely

Felisa

M. Felisa Puche Pinazo

Departamento de Botanica

Facultad de Ciencias Biologicas

tel. 96 354 4633

Universitat de Valencia

Dr.Moliner n 50, 46100- Burjassot

Valencia. Spain

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Thanks you very much of the interesting discussion on the use of the term "Parasite".

I personally agree that the sporophyte is, at least partially, a parasite of the gametophyte. In human body, hands are not parasites as they are part of the body, whereas in plants, gametophytes and sporophytes are completely different entities. As one is haploid other is diploid, the genetic make up is at least partly different. Consequently, I would consider animal fetus a parasite on its mother. I think that is a evolutionary strategy to nurse the next generation.

Animals that hunt and eat its own species are still consider carnivores (although it is a special kind of carnivory call cannibalism), why can't we call a species a parasite when it get its nutrition from another individual of the same species? Unless someone wants to invent a new term for this special kind of parasitism of plants.

Best wishes

Boon-Chuan Ho

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

*German website, referring to bryophytes, states "If

these both wouldn't belong to the same species, the sporophyte could be

called a parasite on the gametophyte."*

The question is a bit semantic and perhaps even philosophical, but I think

the above statement is correct. How can a sporophyte be defined as a

parasite of a gametophyte, if the sporophyte's whole existence only aims at

producing more gametophytes like the one it is sitting on? True, the new

gametophytes will not be genetically identical with the original one, but

that is not the point here, particularly since the gametophytes of many if

not most bryophytes are capable of producing clones of themselves if and

when they "have to". I think the term parasite is wrongly applied here.

Best regards,

Johannes

--

Dr. Johannes Enroth

PhD, University Lecturer, Bryologist

Dept. of Biological and Environmental Sciences

P.O. Box 65

FIN-00014 University of Helsinki

Finland

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Dependant vs parasitic?

Is this distinction fouynded on whether the sporophyte actively enforces a

regieme of growth and development onto the gametophyte, the result of

which is the nutritional interface and protective structures produced by

gametophyte.

Does the gametophyte (as does any mother) actively facilitate the growth

and development of the sporophyte? Does a chemical stimulus from the

sporopyte initiating a signal transduction cascade (or whatever transmits

that signal) in the sporophyte constitute the same kind of enforcement as

a tick sticking its mouthparts into you?

I would argue that the relationship between sporophyte and gametophyte

stands in sharp contrast to the relationship between host and parasite,

regardless of nutritional similarities in both types of relationship.

regards

Matt

_______________________________________

Matt Renner

PhD candidate

The Wardle Lab

School of Biological Sciences

Heydon Laurence Building A08

University of Sydney, NSW 2006

(02)9351 5118

041 585 2205

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

There is a whole generation of my high school students who learned a rhythmic "choral reading"  of a short poem that went like this:

 

In (pause) mosses,

the sporo-phyte

is a para-site

on the fe-male

gameto- phyte.

 

I stood at the front and waved my arms around like a choir director as we said this over and over. They were very young high school sophomores, and I did whatever I could to keep them focussed and have a little fun along the way. 

 

They were also quite attentive after I told them that mosses and ferns can only do it in the rain....smile.

Sadly, with the advent of standardized state tests and therefore, standardized curriculum topics, there is no longer time to teach a survey of either the plant or animal kingdoms. I guess when we retire we're allowed to reminisce about the good old days.

Maggie Ray

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Perhaps the term hemi-parasite may better sum up the situation in most,

but not all, bryophytes. I think the situation in mosses may be more or

less unique.

In Marchantia, or other marchantioid hepatics, the sporophyte undergoes

much of its development without the apparent need for photosynthesis.

Riccia is perhaps the extreme case but even in Asterella, Marchantia,

Lunularia, Reboulia, the carpocephalum contains photosynthetic tissue

but it may be of use (to the sporophyte itself) only in the final stages

where the sporophytes are elevated above the thallus.

Jungermanniales do not, as far as I am aware, have photosynthetic tissue

in their sporophyte.

I wouldn't consider my arm, leg, head even, parasitic on the rest of me

because each part derives all it needs (I think!) from the whole.

If one looks at the case of Buxbaumia, what do we call the maturing

sporophyte - which is very much larger and more conspicuous than the

gametophyte (which, I hasten to add, I have never actually examined or

seen).

If one looks at Ephemeropsis, and perhaps to a slightly lesser extent

Viridivellus, where the gametophyte is about the size of the sporophyte

or may, because it is like a protonemal felt, be the dominant part, we

have perhaps a 50:50 situation.

In many instances the moss sporophyte is the lesser party. But, if we

have to draw analogies because our minds like to have comparisons or

places in which to put things, I think the instance of the animal foetus

is probably the equivalent of the situation in a bryophyte - no, moss -

sporophyte.

In hepatics, the sporophyte perhaps comes nearer to a truly parasitic

relationship - but this then gets back to the philosophical argument

about the possibility of being parasitic on oneself.

The hornwort sporophyte is perhaps nearest to independent existence, but

it depends on sperm (possibly foreign in the sense of being from another

plant) being subsumed in nuclear fusion by the gametophyte egg cell to

form the zygote.

Perhaps not entirely unrelated to this discussion is the thesis once put

forward that the chloroplast was originally a foreign organisms captured

by another and subsumed in the dominant partner's make up. There is

chloroplast DNA after all. Perhaps the molecular people can answer the

question - how different is the chloroplast DNA to, nuclear DNA or

ribosomal DNA?

Regards,

Rod Seppelt.

Prof. Rod Seppelt,

Australian Antarctic Division

Channel Highway,

Kingston, Tasmania 7050,

Australia

ph: +61 (03) 62 323 438

FAX: +61 (03) 62 323 449

email: rod.seppelt@.au

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Why not a hemi-parasite?

 

I wouldn't consider my arm, leg, head even, parasitic on the rest of me because each part derives all it needs (I think!) from the whole.

If one looks at the case of Buxbaumia, what do we call the maturing sporophyte - which is very much larger and more conspicuous than the gametophyte (which, I hasten to add, I have never actually examined or seen). 

If one looks at Ephemeropsis, and perhaps to a slightly lesser extent Viridivellus, where the gametophyte is about the size of the sporophyte or may, because it is like a protonemal felt, be the dominant part, we have perhaps a 50:50 situation.

In many instances the moss sporophyte is the lesser party.  But, if we have to draw analogies because our minds like to have comparisons or places in which to put things, I think the instance of the animal foetus is probably the equivalent of the situation in a bryophyte - no, moss - sporophyte.

In hepatics, the sporophyte perhaps comes nearer to a truly parasitic relationship - but this then gets back to the philosophical argument about the possibility of being parasitic on oneself.

The hornwort sporophyte is perhaps nearest to independent existence, but it depends on sperm (possibly foreign in the sense of being from another plant) being subsumed in nuclear fusion by the gametophyte egg cell to form the zygote. 

 

Prof. Rod Seppelt,

Australian Antarctic Division

Channel Highway,

Kingston, Tasmania 7050,

Australia

ph:  +61 (03) 62 323 438

FAX:  +61 (03) 62 323 449

email:  rod.seppelt@.au

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Parasitism? I would say: no.

 

The problem of course lies in the definition of 'parasitism'. The one given here uses the 'plant or animal' (as if nothing else would exist, and I think here you can clearly see how traditional and conventional this approach is) as the unity. May be, we should think about the sporophyte and the gemetophyte as expressions of one and the same genome, and, in a sense, parts of a same 'organism'?

 Hans de Bruijn

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Its an interesting question. I checked the mening of parasitism by wikipedia:

I think for bryophytes this word can not be used because: it should be between phylogentically different individuals and the parasite must cause harm to the host:

("It is important to note that "benefit" and "harm" in the definition of parasitism apply to lineages, not individuals. Thus, if an organism becomes physically stronger as a result of infection but loses reproductive capabilities (as results from some flatworm infections of snails), that organism is harmed in an evolutionary sense and is thus parasitized.")

Since the relationship between the gametophyte and sporophyte is a necessity for reproduction (well...hm. depending on if the species use sexual or vegetative reproduction) it is an evolutionary benefit.

Perhaps its time to come up with a new term for the special relationship between gametophytes and sporophytes in bryophytes (a relationship that causes temporary harm to the host but is an evolutionary benefit in the longer term). But on the other hand: it is very similar to other cases as you already mentioned. Cases that are not considered to be parasitic.

So, I do not know. Unfortunately, the world is usually much more complicated that systematist and language allows it to be.

Annika Jagerbrand

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Perhaps the  term "hemiparasitic" could be better appropriate?

René Schumacker

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

This is interesting but I think you have already answered the

question with what you presented. Maybe I'll make a misstep here, so

anyone correct me if I do. Since the gametophyte always gets the

benefit of sexual reproduction from the sporophyte, and since the

gametophyte produces the cells that lead to the creation of a

sporophyte, I simply can't see how the term "parasitic" applies to a

sporophyte. In essence, I don't see how one part of an organism is

parasitic on another part. As mentioned before, a fetus (imperfect

analogy since that is more like asking if a spore is parasitic on the

gametophyte - both are the same generation, and still, as you well

know the human placenta is not of the opposite generation from the

mother so it is even a weaker example than a sporophytic placenta) is

not a parasite and neither is a tumor. A tumor is also an imperfect

analogy, but closer to a parasite in my mind in that there is no

reproductive benefit to the host (at least in most cases). However,

the tumor is NOT another organism and therefore, I would argue that

it cannot be considered a parasite. I understand the desire to use

parasitic because it is provocative, but I think the term "dependent"

is appropriate and that we should be clear to make this distinction.

If the concern is how to characterize sporophytes that can provide

some photosynthate for themselves, I would use the term

"semi-dependent". As for the the chloroplast being enslaved by the

cell after endosymbiosis, one should realize that it is a two-way

street benefiting both former organisms - a real mutualism. To

partially answer Rod's question about differences in DNA, I believe

the G:C content is quite different, but more fundamentally (in my

mind) the chloroplast genome is a circuluar plasmid (just like

bacteria) rather than a set of chromosomes (like eukaryotes) and the

promoter structure is very different between nuclear and chloroplast

genomes. I couldn't be more convinced that the chloroplast is

essentially a bacterium within a eukaryote, but I won't go into all

the various sorts of evidence.

-Dave Hanson

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

I believe that the term parasite reflects the mode of life of a species in relation to a different species. This is a species fitness issue having population effects  "In the population interaction called parasitism, one population (the parasite) benefits, and the other (the host) is harmed by the relationship."

Because the plant sporophyte is just a different life phase of the same species as the gametophyte, their relationship is no more parasitic than that of a eutherian mammal's fetus. I both cases (and in the florideophycean red algae and cases of viviparous non-mammals)  the fitness of the species is improved by maternal provision of nutrients to the young. I call this "plant matrotrophy," a term first used in discussions of viviparity in animals.

The classic experiments of Adrian Browning and Brian Gunning demonstrated experimentally that matrotrophy improves fitness in Funaria hygrometrica, a result that can reasonably be extrapolated to all embryophytes (in the absence of evidence to the contrary).

I would say that the plant sporophyte is matrotrophic on the gametophyte for at least some period of time during early embryo development.

Linda Graham

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

It sounds like a tricky question if the sporophyte is parasitic or not, in

bryophytes. I think Ros Seppelt gave a nice introdution to the differences and

problems involved, thank you. In any case, I also would exclude parts of the

same organisms (and same generation) in the definition of parasitic.

Regarding chloroplast, generally three things are indicated as supporting an

endosymbiotic origin of it, a) a double membrane, b) and more interestingly, a

DNA which is of the prokaryotic type (no istones), and, c) ribosmes of the

bacterial type (different from those of eukaryotic cells), even more

interestingly, the same reasons apply to mitochondria too.

--

Dr Carmine Colacino

Lab. di Briologia - Dip. di Biologia DBAF

Universita' della Basilicata

Viale dell'Ateneo Lucano, 10

Potenza, Italy

Ph. +39-0971-206234; Fax +39-0971-206233

Lab.+39-0971/205346: Mob.+39-329-3178399

Alt. e-mail: colacino@bryology.eu





++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The assertion of a parasitic sporophyte assumes that there is no benefit for

the gametophyte from the sporophyte. If one is to accept one of the basic

tenets of evolution...the primary goal is the survival of the DNA into the

future..., then the question seems to answer itself. The sporophyte is

preserving the genetic code of the gametophyte by producing spores. The

spores can survive inclement conditions, or even be transported in ways the

gametophyte cannot.

Obviously, bryophytes, with their infinite variety of asexual reproductive

strategies, do not rely on sexually produced offspring. However, the fertile

gametophyte spends a great deal of energy creating this "parasite".

Extending the human metaphor, the "parasite" child is not only an energy

drain in utero, but also for the first 15-20 years of life! This is an

awesome investment of energy. It is obvious that the species, and thus the

DNA would perish without offspring, and is thus clearly beneficial to the

parent.

So parasite is at best an inaccurate word for this relationship. Partner

seems a better place to start if one is convinced that the two generations

constitute a separate being.

Ken Kellman

9870 Brookside Ave

Ben Lomond, CA 95005

kkellman@

(831) 336-8548

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

On "parasitic", I tend to think that this is a bit harsh to apply to moss sporophytes, since the sporophyte represents offspring, and the gametophyte allocates nutrients toward their maturation, actively mobilizing their transfer as I'm sure you are aware through an elaborate system of coevolved membranes at the sporophyte/gametophyte juncture. When stressed, I'm finding that the maternal gametophyte jettisons the sporophyte to preserve herself, and this is to be expected in perennial species, which then live to reproduce another set of offspring hopefully. Clearly, the two generations signal each other, and the fitness of one is intertwined with the fitness of the other. I'm reminded of the stimulation of postfertilization perichaetial leaves and the function they may confer in protecting the young embryos of many pleurocarpous mosses (a phenomenon not expected in a parasitic situation).

As for an alternative term, would your suggestion of "hemiparasitic" work? To me the analogy is more to a fruit on a tree. Maybe anything dealing with "parasitic" really connotes too negative for what is going on, however at the moment I have nothing to put forward, perhaps something will come to me. When I think about the generations and watch them grow, I tend to think of a single entity with two organs, rather than two separate entities vying for survival.

Lloyd Stark

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Yes, I think that you have to consider the sporophyte as a parasite on the gametophyte. Considering Claudio's points I would say that the situation is different to the hand and the body because the sporophyte is genetically distinct from the gametophyte. Also, nobody doubts that mistletoe (Viscum album) is parasitic on the host tree, but it appears to do the tree little or no damage and does also photosynthesise.

 

I think that what makes us feel uncomfortable is that the sporophyte is of some benefit to the gametophyte in that it does disperse its genes. Perhaps we could say that the sporophyte is a physiological parasite but a genetic symbiote, or is that too much of a mouthful?

 

Jonathan Sleath

++++++++++++++++++++++++++++++++++++++++++++++++++++++

I shouldn't put the idea into university heads, but as an undergraduate at Queen's University in Kingston, Ontario in the 1960's I had to undergo a final oral (as well as written) examination.  So the question of whether or not the sporophyte is parasitic would be a wonderful question for a student.  It is entirely open-ended and would certainly tell if the student could think rationally and quickly on his or her feet.  One question I was asked during my oral was whether or not viruses were alive. Nasty stuff.  About the sporophyte - I'll leave that debate to those more knowledgeable than I!  Allan Aubin

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download