In this article we will discuss about:- 1. History of Heterospory 2. Incipience of Heterospory 3. Origin 4. Evidences 5. Advantages.
History of Heterospory:
Pteridophytes are the first true land plants. Being the first successful colonisers in land habit pteridophytes show both the homospory and heterospory. Most of the pteridophytes are homosporous, produce isospores which are morphologically and physiologically identical.
On exosporic germination the spores produce monoecious gametophytes bearing both antheridia and archegonia. While, heterospory has been reported in nine genera namely, Selaginella, Isoetes, Stylites, Pilularia, Regnellidium, Marsilea, Salvinia, Azolla and Platyzoma. Heterosporous forms produce two kinds of spores: microspores produced in microsporangia and megaspores within megasporangia.
Microspores germinate to produce male gametophytes, while megaspores give rise to female gametophytes. Both the spores germinate endosporically to produce gametophytes. The existence of heterospory has importance in the origin and evolution of seed. Thus the origin of the seed habit is a logical progression that began with homospory, was followed by stages of heterospory, and culminated into the seed.
Incipience of Heterospory:
It has been universally accepted that the homosporous condition is primitive and heterospory is derived. The sex determination in homosporous forms might have occurred where some of the isospores produced either exosporic male or female gametophyte.
Thus, heterospory might have first appeared in plants where each sporangium contained both microspores and megaspores. So one might expect some conditions showing incipient heterospory.
Although Equisetum is a homosporous pteridophyte, sometimes it produces two types of gametophytes — male prothallus is smaller in size, while female one is larger. This sex differentiation appears to be related to environmental condition.
Some of the population of male gametophytes remained male throughout the successive generations, while the larger population bearing archegonia become hermaphrodite if fertilisation is delayed. So Equisetum gametophytes are potentially bisexual where sex determination is related to environmental factors, though they did no. lose their male potentiality.
The monotypic Ceratopteris thalictroides is a homosporous leptosporangiate fern, but shows incipient heterospory. The spores are alike but produce two types of gametophytes; the small, strap-shaped, slow-growing male gametophytes and the large lobed, fast-growing female gametophyte. The female gametophyte may bear antheridia if fertilisation fails.
Platyzoma, a northern Australian heterosporous fern, shows anisospory. The plants bear two types of sporangia — the smaller ones produce 32 spores and the larger ones produce 16 spores. The larger spores (megaspores) are twice the diameter of smaller spores (microspores).
Microspores germinate to produce filamentous gametophyte bearing antheridia. While, megaspores germinate to form spathulate gametophyte that bear archegonia. If fertilisation fails the female gametophytes produce antheridia on lobes. So Platyzoma is an intermediate between incipient heterospory and complete heterospory.
Thus from the above evidences it may be concluded that heterothallism might have preceded heterospory in evolution.
Origin of Heterospory:
The fossil evidence indicates that some sporangia of the plant contained both small and large spores, though the size differences are not so pronounced. So a single sporangium contained both microspores and megaspores. At a later period, the microsporangia and megasporangia might have been differentiated.
Microsporangia contained a number of microspores and megasporangia — either on the same plant or on another plant of the same species — contained a small number of megaspores. Pteridophytes are free- sporing, so both the microspores and megaspores with their endosporic gametophytes are shed.
In heterosporous members, there is a tendency to reduce the number of megaspores from many to one functional megaspore. It has been evidenced that in some members, such as Lepidocarpon and Miadesmia, the megaspores with their mega-gametophytes are retained in a dehisced megasporangium.
Stewart and Rothwell (1993) proposed the five stages in the evolution of heterospory from the primitive homosporous condition:
1. Decrease in numbers of spores in some sporangia of homosporous member.
2. Increase in size of the remaining spores in those sporangia.
3. Spore content in those sporangia remain constant in size and number.
4. Change from monoecious to dioecious gametophytes.
5. Change from exosporic to endosporic gametophyte.
Evidences of Early Free-Sporing Heterospory:
Palaeobotanical records show that the earliest vascular plants prior to the Middle Devonian were homosporous and probably produced exosporic monoecious gametophytes. The first definite heterospory was reported in the Upper Devonian. Although, palaeobotanists are trying to find out any evidence of heterospory in strata older than the Upper Devonian.
Chauleria resembles Aneurophytales (Progym- nospermopsida); it was reported from the Middle Devonian. The sporangia of Chauleria contained numerous spores of two sizes, though the size differences are not so pronounced.
Smaller spores (microspores) are 30-40 μm in diameter, while larger spores are 60-156 μm in diameter. Some sporangia contain only microspores, while some other contain a mixture of micro- and megaspores.
The definite free-sporing heterospory was discovered in enigmatic Upper Devonian Barinophyton citrulliforme. The smaller microspores have the size range of 33-48.5 μm, while the megaspores are very large in the 700-900 μm range. Barinophyton citrulliforme is an amphisporangiate member because some sporangia contain both the microspores and megaspores.
Thus, Chauleria and Barinophyton provide evidence about the shift from the isospores of homosporous plants to the microspores and megaspores of a heterosporous plant. Williamson and Scott (1894) reported initial steps of heterospory in Calamostachys, a fossil Carboniferous sphenopsid.
Although Calamostachys is homosporous, in some sporangia the spores are of different sizes. The larger spores are thrice the diameter of smaller spores. C. americ na showed amphisporangiate condition. Palaeostachya andrewsii, the other fossil sphenopsid member, is heterosporous. The microspores have a size range of 56-110 μm; the megaspores ranging from 235-345 μm.
The further important steps in heterospory, i.e. the differentiation of micro- and megasporangia, and reduction in number of megaspores have been revealed in Archaeopteris, an Upper Devonian progymnosperm. True heterospory has been reported in two spices of Archaeopteris (A. halliana and A. macilenta).
Arnold (1939) reported a marked difference in sporangium size, spore dimension and spore content in A. halliana. Here megasporangia are broad, contained 8-16 megaspores up to 300 μm in diameter, while the slender microsporangia contained 100 microspores about 30 μm in diameter.
The further reduction in the number of megaspores has been reported in Stauropteris burntislandica, an Upper Devonian heterosporous coenopterid fern. Here a spindle-shaped megasporangia bore two functional and two abortive megaspores.
The next important step in heterospory, i.e., the formation of a functional megaspore in the sporangium, has been revealed in Lepidocarpon, Miadesmia (Carboniferous lycopods) and Calamocarpon (Carboniferous sphenopsid).
In all the members, there is a single functional megaspore in the megasporangium. The megagametophytes retained inside the dehisced megasporangia and were shed as a unit from the cone. On the basis of the features revealed in the above three members the origin of seed habit can be evaluated.
Biological Advantages of Heterospory:
1. In heterosporous forms, the endosporic gametophytes are independent of external conditions and self-supportive in food supply. Thus, the female gametophyte that developed within the large megaspore, at the expense of reserver food, is in a better situation for the initiation of embryo. Whereas, the free-living exosporic gametophytes of homosporous members are totally dependent on external conditions.
2. Heterospory induces a drastic reduction in the sizes of their gametophytes. There is an extreme reduction in the male gametophyte and, in most cases, it is nothing but an antheridium. The female gametophyte never outgrows the limits of the megaspore.
3. In heterosporous forms the differentiation of sex can be predicted at the spore stages. Thus the sex differentiation can be extended from the early stage of gametophyte to the sporophyte.