Give an illustrated account of the evolution of gametophytes in Bryophytes.

Ans. There are two schools of thought explaining the course of evolution of gametophytes in bryophytes. According to one school of thought, the simple thallus of marchantiales is the result of retrogressive evolution. The other group considers the simple marchantiales have evolved as a result of progressive evolution.

The supporters of this theory are Wettstein (1903, 1908), Church (1919), Kashyap (1919), Goebel (1906), Evans (1930), Mehra (1953, 1957), Proskauer (1960), Fulford (1965), Zimmermann (1966) and Udar (1970). According to this theory the primitive gametophyte was an erect leafy shoot of bryopsida, the gametophyte of acrogynous Jungermanniales were the most primitive as they were similar in many respects to the gametophytes of bryopsida. A reduction in the leaf development resulted in the dorsiventral thalli of acrogynous Jungermanniales, Marchantiales and Anthocerotales.

According to Wettstein (1901-1903), the nearest approach to a hypothetical primitive ancestor is to be observed in the leafy gametophytes of calobryales and many true mosses as Fontinalis, Polytrichum and Bryum and also in the sexual branches of some of the prostrate forms of thallose and leafy Hepaticopsida. From such forms arose dorsiventral thalli of many Hepaticopsida and Anthocerotopsida by retrogressive evolution (progressive reduction).

The derivation of thalloid types from erect radial leafy shoots has been strongly supported by Evans (1939). According to him there was a gradual dimunition in the size of the ventral leaves (Lepidozia) to the extent that they appeared as mere vestiges in the form of slime papillae (Cephalozia) and ultimately disappeared in Radula. Associated with this change, there was a flattening of the axis and a gradual decrease in the size of the lateral leaves to either a few cells or to vestigial marginal slime papillae and finally their complete disappearance in Pellia, Marchantiales and Anthocerotales. Zoopsis and Schiffneria with more or less reduced leaves and flattened axis represent intermediate conditions. A notable feature in many of these cases is that their sexual branches retain the features of typical, radial, erect leafy shoots strongly suggesting that the form of vegetative organ which thus approach the thalloid state is one of secondary origin. In the entire plant kingdom reproductive organs are most conservative structures and the retention of ancestral radial foliar habit by the sexual branches of those dorsiventral, leafless forms can best be explained as due to reduction. The Marchantiales is a typical example where the reproductive shoots-archegoniophores and antheridiophores are conservative in nature. This idea was also supported by Kashyap (1919) by his extensive study on vegetative thalli of Liverworts which have reduced air chambers and pores, while same structures are better development in the disc of archegoniophores and antheridiophores.

Goebel (1906) and Kashyap (1919) are also convinced by the idea of reduction in Liverworts.

Regarding the theory of retrogressive evolution some undermentioned points are important :

(i) Loss of assimilatory filaments in the air chambers: Starting from the genera growing in moist soil to those occurring in water there is gradual loss of assimilatory filaments. The thallus of Preissia quadrata, which grows in moist soil has the characteristics of complex form with air chambers, pores and assimilatory filaments. In conocephalum conicum which grows in comparatively more moist places, the assimilatory filaments show a decrease in size and are short in the greater part of the air chambers. Further in Wiesnerella denudata occurring either near or under water the assimilatory filaments are reduced to papillate cells inside the air chambers. The remaining stage of reduction in assimilatory filaments are seen in the genus Dumortiera. The plants grow normally in water and have complete absence of air chambers, but thalli growing in comparatively less moist soil, the chambers are present at the growing points. The pores are rudimentary. The reduction of air chambers is best regarded as due to their secondary migration to water and such forms could hardly be regarded as the starting point of progressive series.

(ii) Simplification of pores: In complex types as Marchantia and Presissia the pores are barrel-shaped and both on the thallus and the disc of archegoniophores and antheridiophores. In Conocephalum and Reboulia the pores are barrel-shaped on the disc but simple on the thallus. In Exormotheca and Stephensoniella simple pores exist both on the thallus and the disc while in Riccia there are no well defined pores.

(iii) Gradual shifting of sex bearing stalk: The highly complex forms as Marchantia have antheridia and archegonia borne on stalked antheridiophore and archegoniophore respectively which are usually terminal. In other forms they are dorsal. Thus the dorsal position is secondary one. Further reduction is seen in Corsinia where the female receptacle becomes sessile by the elimination of the stalk.

Harris (1938, 1939) on the basis of the studies made on fossils Hepaticopsida postulates that at a certain stage the primitive Hepacae had an erect leafy shoot with spirally arranged leaves as in the moss.

The knowledge of genetics on Marchantia also lends support to the theory of Retrogressive evolution (Burgeff, 1943). Several mutations of Marchantia resembled the phyletic reduction series and showed co-relation with forms as Dumortiera etc. Burgeff was also able to attain in his mutated forms the reduction of high stalked archegoniophore like those of Marchantia to sessile receptacles like those of Corsinia.

Kashyap on the basis of studies postulated that the Marchantiales probably arose by reduction from the pteridophytean ancestors.

Mehra (1957) has raised objections to this hypothesis. According to him there is no any resemblance between the sporophyte of Equisetum or Lycopodium with the sporophyte of simple liverworts.

Condensation theory: Mehra (1957) has postulated the derivation of Marchantiaceous thallus from foliose Jungermanniales through compaction and condensation. His theory is widely accepted and further strengthened by spore morphology in liverworts.

The supporters of this hypothesis are Cavers (1910), Campbell (1918, 1940), Fritsch (1935), Bower (1908) and Smith (1955). According to this theory the present day Hepaticopsida originated from a dorsiventral, prostrate thallus that showed no differentiation externally. Cavers suggested sphaerocarpos whereas Campbell thought forms like Reccardia and Metzgeria as the present day Hepaticopsida whose thalli show a nearest approach to the simplest primitive gametophyte. From such simple thalli, the evolutionary advance proceeded into two diametrically opposed directions and resulted in the formation of two different types of gametophytes. The first line led to the evolution of Marchantiaceae gametophyte and the second line led to the evolution of forms in the Jungermanniales and Calobryales in which the gametophytes retained its simple structure but there was a gradual and progressive elaboration of external form which finally resulted in the formation of a leafy shoot. The leafy forms attain its fullest expression in the higher Bryopsida where it is associated with a higher degree of internal differentiation.

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