Evolution of the male gametophytes of gymnosperms

Evolution of the male gametophytes of gymnosperms

1. EVOLUTION OF THE MALE GAMETOPHYTES OF GYMNOSPERMS

2. PRESENTED BY :- PATEL CHARMI JASHVANTBHAI M.Sc. ( BOTANY ) SEM :- 1 PAPER CBO :- 402 DEPARTMENT OF LIFE SCIENCES , H.N.G.U. PATAN

3. CONTENTS INTRODUCTION STRUCTURE OF MALE GAMETOPHYTE EVOLUTION OF MALE GAMETOPHYTE

4. INTRODUCTION DEFINATION OF EVOLUTION The process by which different kinds of living organism are believed to have developed from earlier forms during the history of the earth. The gradual development of something.

5. The male gametophytes of modern gymnosperms differ appreciably from those of the homosporous pteridophytes in being endosporous, much reduced and in possessing a siphonogamous outgrowth called the pollen tube. The male gametophyte is an outcome of the germination of the microspore or the pollen grains. The germination is partially precocious and partially it takes place on the nucellus of the ovule.

6. It is an extremely reduced structure as compared to the massive gametophytes of the homosporous bryophytes and pteridophytes. It is devoid of any orderly arrangement of cells as compared to the antheridia of mosses and pteridophytes. Some sort of comparison can be made with the male gametophytes of heterosporous pteridophytes like selaginella and isoetes.

7. Both these pteridophytes have completely endosporic male gametophytes with one prothallial cell and an antheridium with 256 antheriozoids in selaginella and only four in isoetes. Liebig (1931) pointed out a sort of similarity between the male gametophytes of isoetes and some fossil gametophytes like cordainthus. In cordainthus , Florin (1936) described a row like disposition of spermatozoids and a tendency towards a similar disposition is evident in isoetes.

8. Sewards and ford (1906) also pointed out towards a resemblance between the organisation of male gametophytes of such heterosporous pteridophytes and some living and fossil A raucariaceae. Structure of male gametophytes of fossil gymnosperms is not completely known for all fossil groups. No data are available regarding the fossil remains of fossil microgametophytes of coniferophytes of carboniferous and permain periods.

9. Well preserved male gametophytes of cycadofilicales and cordaitales are available and all such specimens show multicellular nature of the microgametophytes. Germinated pollen grains have been described in the nucellus of ovules of many genera of cycadofilicales. Oliver (1904) and Renault (1885) described the pollen grains and microgametophytes of the fossil genus stephanospermum.

10. In some specimens these cells have been seen to form a parietal layer enclosing a central cavity. In S.caryoides he reported two cells lying in this cavity and a single peripheral cell enclosing them partially. The peripheral cell was regarded by oliver to be comparable to a tube cell of the modern gymnosperms. The central two cells perhaps divided further to produce antherozoid mother cells.

11. Hoskins and Cross and Stewart studied the male gametophytes of pachytesta vera, P. hexangulata, P. illionensis and P. composita. The male gametophytes have many peripheral cells enclosing a number of internal cells lying in the central cavity of the microspore. Renault (1902) described a small protuberance on one side of the microspore of Aetheotesta and Stephanospermum. He interpreted it as the base of the pollen tube.

12. Florin (1936) described the germinated pollen grains of Cordainthus and reported the presence of definite peripheral cells enclosing a central space containing a row of about five nuclei along the polar axis of the microspore. Florin compared the peripheral cells to the male prothallial cells of living gymnosperms and the five nuclei to the gamete nuclei. These germinated pollen grains were found in the microsporangia.

13. In Cycadeoidea Wieland (1906) found germinated pollen grains in microsporangia, they also contained 1-5 peripheral cells that were interpreted as prothallial cells. The fossil genera completely lacked a pollen tube. A careful study of the microgametophytes of modern gymnosperms reveals that there has been a gradual reduction in the size and content of the male gametophyte. In Ginkgo and Welwitschia the male prothallial cells are altogether wanting.

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