In this article we will discuss about the place of meiosis in oomycetes.
It is still under dispute. Stevens as early as 1899 suggested gametangial meiosis in Albugo. His interpretation was disputed by later workers. Since then it has been widely held that the Oomycetes, as a whole, are haploid and meiosis occurs in the oospore. Sansome (1963) working with Pythium debaryanum struck a discordant note.
She reported that the two successive divisions which occur in the gametangia constitute meiosis. From this one would conclude that the Oomycetes, as whole, would be diploid.
During the last two decades various investigators have reported meiosis in the gametangia of Phytophthora, Albugo and Peronospora, Sclerospora, Bremia, Achlya, Saprolegnia, and Apodachlya. From the investigations of these workers it would appear that both the Saprolegniales and Peronosporales are diploid.
On the other hand, Timmer, Castro, Irvin, Belsar and Zentmyar (1970) advanced a genetic evidence for zygotic meiosis in Phytophthora capsici. Stephenson working with Phytophthora capsici vehemently opposed gametangia meiosis in Phytophthora. They reported that meiosis takes place in the first divisions of the oospore nucleus.
More recently Sansome (1976) studied meiosis in Phytophthora capsici. She observed pachytene, diplotene and diakinesis stages in the oogonium. She further reported that metaphase and anaphase were followed by a second division without any increase in nuclear and chromosome size between the two divisions.
The haploid chromosome number was reported to be nine. She observed an association of 4 chromosomes in one or both parents.
The association of 4 chromosomes at diakinesis and metaphase provides an evidence that pairing of homologous parts occure during the first division and is a proof that the gametangial divisions are meiotic.
There is thus overwhelming and conclusive evidence in favour of gametangial meiosis in the Oomycetes according to which the somatic of gametangial meiosis in the sex organs (antheridia and oogonia) would be diploid the gametes alone represent the haploid structures in the Oomycete life cycle. In spite of this the mycologists, in general, stick to the view that there is zygotic meiosis and Oomycetes are haploid.
Inter-relationship and origin of Oomycetes:
No close relationship seems to exist between the biflagellatae (Oomycetes) and uniflagellatae. In their mycelial and hyphal organisation of somatic phase, cellulose cell wall and oogamous sexual reproduction, the Oomycetes indicate some resemblance with the Monoblepharidales.
The resemblance, however, appears superficial because two groups have totally different types of swarmers.
Among the Oomycetes themselves, the biflagellate condition of the swarmer with two unequal flagella, the shorter of tinsel and longer of whiplash type, oogamy, transfer of male gamete into female by the fertilisation tube and germination of zygote by a germ tube are some of the common features which all point to the close relationship between the members of the Saprolegniales-Peronosporales series.
The Saprolegniales are considered primitive and Peronosporales advanced.
In the siphoneous, coenocytic acellular organisation of the thallus, chemical composition of cell wall, oogamous sexual reproduction, and synthesis of amino acid lysine from diaminopimelic acid, the Oomycetes exhibit a striking resemblance with the Xanthophyceae (Yellow green algae).
In their bilateral symmetry, the swarmers of Oomycetes with two anteriorly inserted unequal flagella, the shorter with flimmer hairs and the longer of whiplash type are suggestive of those of the Heterosiphonales.
The recent discovery of tinsel and whiplash types of flagella in the motile stages of Vaucheria has given fillip to the belief that the Oomycetes could have evolved by loss of chlorophyll from the Vaucheria-like ancestor and probably before it had developed its compound zoospores from many biflagellate zoospores.
Whether these resemblances are suggestive of real affinity or the result of converging development is difficult to decide.