Cytologia 51: 117-124, 1986

The Distribution and Systematic Significance of Pollen Nuclear Number in the

P. M. Mathew and Omana Philip

Department of Botany, Kerala University, Kariavattom, Trivandrum,

Accepted September 22, 1984

In angiosperms, the mature pollen grains are either binucleate or trinucleate, the condition being fairly constant within a , and usually within a or entire (Brewbaker 1967). The essential difference between the two types rests on the time of division of the generative nucleus, which in the trinucleate grains take place in the grain itself before dehiscence, and in the binucleate grains this is delayed, and takes place in the pollen tube. Information on pollen nuclear number in angiosperms has been known to be of value in dealing with their systematic relationships and phylogeny (Schurhoff 1924, Schnarf 1937, 1939, Brewbaker 1967, Webster and Rupert 1973). Although most systematists have treated the Rubiaceae as occupying an ad vanced position among the families of flowring , considerable difference of opinion exists as regards its systematic relationships and delimitation of taxa within the family. Previous information on the pollen cytology of the Rubiaceae is strik ingly meagre, and the available data are chiefly on temperate taxa (Brewbaker 1967). During the cytopalynological investigations on the South Indian Rubiaceae recently undertaken by us, pollen cytology of a large number of species from this region was studied with a view to determining the distribution of the two nuclear types in the group, and to use the data as an aid in the systematic consideration of the family. The data presented here covers 103 species representing 32 genera and 15 tribes.

Material and methods

Most of the species investigated here were collected from different low and high altitude regions of the Tamil Nadu and Kerala section of the Western Ghats in S. India. A few species from the plains as well as the sea coasts of the Kottayam, Alleppey and Quilon districts of Kerala State were also studied. Pollen nuclear preparations were made following the technique suggested by Maheshwari and Wulff (1937), by which the pollen grains were stained in acetocarmine commonly without prior fixation. In the case of species with thick-walled grains, the modified method suggested by Rupert and Webster (1972) was followed.

Observations

Data of distribution of the binucleate (II) and trinucleate (III) pollen grains 118 P. M. Mathew and Omana Philip Cytologia 51

Table 1. Data of pollen nuclear number, basic chromosome number and pollen aperture number in species belonging to 32 genera of the Rubiaceae from South India

among the South Indian members of the family are given in Table 1, listed according to Hooker's (1880) order. Available information on the basic chromosome num bers (Philip 1978) and pollen aperture numbers (Mathew and Philip 1983) of the group are also provided in the Table to be used as supplementary evidence in discussing the systematic relationships of the family. Of the 103 species reported here, 77 are binucleate and the rest trinucleate. The number of pollen nuclei in them showed remarkable constancy within individual tribes and genera except the tribe Hedyotideae and the genus Oldenlandia. Out of the sixteen species of Oldenlandia examined, seven were binucleate and nine tri nucleate. In great bulk of both the binucleate as well as trinucleate species, the vegetative nucleus, although first appeared as a well defined structure, was found 1986 Pollen Nuclear Number in the Rubiaceae 119 to undergo degeneration during the course of development of the microspore , becoming diffuse and refractory to stain in mature grains. Among the binucleate species, the vegetative nuclues persisted till anthesis in only one species (Randia gardneri), and among the trinucleate ones this was noticed in three species (Borreria eradii, Galium asperifolium, cordifolia). The generative nucleus was charac teristically smaller and darkly staining. In most species this nucleus was spherical at dehiscence, and in a few they were spindle-shaped or elongated. The sperm nuclei showed intense staining, and were spherical or ellipsoidal. Out of the total 23 binucleate genera reported here, all except three possessed 3-aperturate grains, while six out of the eight trinucleate genera had six or more apertures.

Discussion

The phylogenetic significance of pollen nuclear number in angiosperms was first recognised by Schurhoff (1926) who proposed that taxa with trinucleate pollen are phylogenetically advanced compared with related taxa with binucleate grains. More recently Brewbaker (1967) who studied the pollen cytology of a large number of angiosperm families confirmed the speculation made by Schurhoff, and showed that angiosperm genera are in general monotypic with respect to pollen nuclear number, either binucleate or trinucleate. Out of the 32 genera of the Rubiaceae examined from South India, 23 are binucleate, eight trinucleate and one genus (Oldenlandia) heterotypic. In no instance did both the conditions coexist within a single species, and it appears that the two nuclear types in the family are mutually exclusive. In many angiosperm species the vegetative nucleus is reported to undergo degeneration during the course of microspore development such that this nucleus is invisible in mature grains. In 63 out of the 64 binucleate and 32 of the 39 tri nucleate species reported here, this nucleus is degenerative. The apparent absence of the vegetative nucleus did not, however impair pollen germination and tube elongation in them. Even in species of where the vegetative nucleus got eliminated from the developing microspore well before dehiscence (Philip and Mathew 1975), pollen germination and tube growth were very normal. The species reported here appeared to show some recognisable association between pollen nuclear number and pollen aperture number such that the binucleate species are 3-aperturate and trinucleate ones predominantly 6 or more aperturate. Similar association has earlier been reported in the Labiatae (Erdtman 1952). Ac cording to Brewbaker (1957), the number of furrows in the pollen grains is possibly determined at the quartet stage. He considers that occurrence of the first mitotic division during the quartet stage produces 6-colpate grains, while microspores in which the first division occurs after the spores have separated become 3-colpate. He has contended that the 3 and 6-colpate conditions in the mint family represent late and early division respectively of the microspore nucleus. The strong asso ciation between the nuclear types and the corresponding pollen aperture condition referred to above in the Rubiaceae may be suggestive of a non-random distribution 120 P. M. Mathew and Omana Philip Cytologia 51 of the binucleate and trinucleate condition in the family. Comparable association between pollen nuclear number and certain other characters has been known in the Euphorbiaceae (Webster and Rupert, 1973). The present group showed yet another association also, namely growth habit and pollen nuclear number. Here, the tri nucleate condition occurs almost exclusively among the herbaceous genera distrib uted in different tribes such as the Hedyotideae (Anotis, Hedyotis and some species of Oldenlandia), (Borreria, Mitracarpum, Richardia), () and Rubieae (Rubia, Galium). This perhaps may be considered to indicate that the shift from the binucleate to trinucleate condition in the family was concomitant with overall specialisation of the habit.

Systematic considerations Of the various older classifications of the Rubiaceae, Bentham and Hooker's (1873) is the most comprehensive one, while Bremekamp's (1966) treatment is the most elaborate among the modern ones. There exists several points of agreements as well as certain obvious disagreements between these two classifications, especially as regards the subdivision and systematic position of certain tribes. This aspect, and also the affinities of the family are considered in the light of evidence from pollen cytology taken together with relevent chromosomal and palynological data on the group.

Tribes Naucleeae and Cinchoneae According to Bentham and Hooker, the Naucleeae occupies the most primitive position in the family, the tribe Cinchoneae being placed next to this. Bremekamp, who also considers the Naucleeae and Cinchoneae to be primitive tribes has, how ever assigned the Naucleeae a position next to his Cinchoneae. Further, he has separated genera such as Anthocephalus, Stephagyne and Uncaria from Bentham and Hooker's Naucleeae, and transferred them to the Cinchoneae on account of the capituliform inflorescence possessed by members of these genera. It may be noted that all the cytologically known genera belonging to these two tribes have the same pollen nuclear situation (binucleate) and chromosome constitution (x=11) except Cinchona with x=17, which in turn is a derivation from x=11 by secondary hybrid polyploidy (Philip 1978). Furthermore, palynologically all the known genera of the two tribes exhibit the same pollen aperture morphoform (3-colporate), and hence these evidences do not appear to contribute favourably towards Bremekamp's separation of the genera mentioned above from Bentham and Hooker's Naucleeae to Cinchoneae.

Hedyotideae As regards the delimitation of this tribe there is great deal of disagreement between the classifications of Bentham and Hooker and Bremekamp . According to the latter's consideration, the tribe Hedyotideae should comprise only genera which show valvate aestivation of the corolla lobes, pluri-ovular ovary cells, peltate placenta attached to the middle of the dissepiment. He has retained in his Hedyoti deae only such of the genera which show these features (Dentella, , Hedyotis, 1986 Pollen Nuclear Number in the Rubiaceae 121

Oldenlandia), the rest having been separated and treated under different tribes such as Argostemmatideae (Argostemma, ), Cruckshanksieae (Antis) and (Ophiorrhiza). It is seen that both cytologically and palynologically Bentham and Hooker's Hedyotideae constitute a very heterogeneous group . With respect to pollen nuclear number this is the only tribe in the entire family which show both bi-and trinucleate pollen grains. Moreover, this tribe shows a wide range of basic chromosome numbers (x=6, 7, 8, 9, 10, 11, 13), the like of which is not met with in any single tribe in the family. Again the tribe is highly euripalynous (3-4 colpate, 7-10 colpate, 3-colporoidate, 3-colporate). On the other hand, with regard to all these features, considerable homogeneity is noticed within each of the four tribes recognised by Bremekamp for Bentham and Hooker's Hedyotideae. As to the systematic position of the Hedyotideae also there is noticeable dis agreement. In Bentham and Hooker's classification this tribe is kept in Series A, which apparently does not represent an advanced position in the family. Verdcourt (1958) also has made a similar treatment by placing all the four tribes referred to above in his which occupies a relatively primitive position (Sub family I). In Bremekamp's classification also these tribes, except Ophiorrhizeae, are placed in the Sub family Rubioideae, which according to him, however occupies a very ad vanced position (Sub family VII). All the known genera of these three tribes show advanced cytological (trinucleate pollen grains and chromosome numbers which are derivatives of x=11) and palynological features. The Ophiorrhizeae has been treated by Bremekamp at a relatively primitive position by keeping this in his Uro phylloideae (Sub family II). Cytological situation in the type genus Ophiorrhiza (binucleate pollen and x=11) does not evidently represent an advanced condition, and hence the placement of the Argostemmatideae, Hedyotideae and Cruckshank sieae in an advanced position and the Ophiorrhizeae in a primitive position as done in Bremekamp's classification seems appropriate.

Other tribes As to the placement of genera of most of the remaining tribes, especially the Guettardeae, Knoxieae, Vanguerieae, Ixoreae, Morindeae, Psychotrieae, Antho spermeae, Spermacoceae and Gallieae, there is close agreement between the classi fications of Bentham and Hooker and Bremekamp. Available data of chromo some numbers and pollen nuclear number distribution show that within each of these tribes there is remarkable uniformity. As regards the systematic position of most of these tribes, Bremekamp's classification by and large agrees with that of Bentham and Hooker. But Bremekamp's placement of some of the tribes differs considerably from that of Verdcourt (1958) especially the Guettardeae, Knoxieae, Spermacoceae and Rubieae. Verdcourt has placed the Guettardeae in his third Sub family, Guettardoideae which occupies the most advanced position in his classification. But in Bremekamp's treatment this Sub family has only an inter mediate position. According to the available cytological information, the Guet tardeae (x=11 and binuclaeate pollen) does not represent an advanced condition. In Verdcourt's classification the Knoxieae occupies a relatively primitive position, being placed in his first Sub family Rubioideae, whereas in Bremekamp's treatment 122 P. M. Mathew and Omana Philip Cytologia 51 this tribe occupies a very advanced position. The cytological situation (x=10 and trinucleate pollen) appears to justify the latter treatment. Again, in Verdcourt's classification, the Spermacoceae is placed in his first Sub family Rubioideae, while both Bremekamp and Bentham and Hooker have assigned this tribe a very advanced position in the family. All the known genera of this tribe possess trinucleate pollen grains and derived chromosome constitution (x=14, 15, 16), and this apparently supports the advanced placement of this tribe. According to Bremekamp, Bentham and Hooker and also Hutchinson (1969), the tribe Gallieae occupies a very advanced position in the family, while Verdcourt has given the tribe a much inferior position. The trinucleate pollen grains consistently present in the members of this tribe, as well as the remarkably prominent polyploid evolution known in the type genus Galium may be considered as evidence in favour of the advanced status of this tribe in the family. Considerable disagreement exists also concerning the systematic relationship and affinities of the Rubiaceae. Bessey (1915) has grouped the family in his Rubiales along with Caprifoliaceae, Adoxaceae, Valerianaceae and Dipsacaceae. Bentham and Hooker (1873) also have placed it in the Rubiales, but along with only two of Bessey's families such as Caprifoliaceae and Adoxaceae. Wagenitz (1959), how ever has placed the family under a different order, the along with a group of five other families such as the Loganiaceae, Apocyanaceae, Asclepiadaceae, Gentianaceae and Menyanthaceae, and has treated the other families of Bessey's Rubiales under a different order, the Dipsacales. Subsequently a few others also favoured this treatment (Takhtajan 1969, Thorne 1976). Based on chemical evi dence, Dahlgren (1975) has supported the inclusion of the Rubiaceae in the order Gentianales, but he considered the Gentianales and Dipsacales as non-separated orders. Available pollen cytological data (Brewbaker 1967) show that all the families of Wagenitz's Gentianales are heterotypic (binucleate and trinucleate), while the families of the Dipsacales are strikingly monotypic (only trinucleate). Further more, all the families of the Gentianales sensu Wagenitz show x=11 as the com monly occurring and predominant basic chromosome constitution, which is partic ularly striking in the Rubiaceae. This is clearly in contradistinction with the situation in the families under the Dipsacales, and hence the Gentianalean affinity of the Rubiaceae appears more natural. Bremekamp (1966) has even proposed to treat the Rubiaceae as an order by themselves, the Rubiales, and to be more closely related to the other families of the Gentianales. Recently Mathew and Philip (1983), based on evidences from pollen morphology, have supported this treatment. Cronquist (1968) who also favours the treatment of the Rubiaceae as a monotypic order, however holds that this family forms a connecting link between the Gentianales and Dipsacales.

Summary

Data on the distribution of binuclaeate and trinucleate pollen grains in 103 species representing 32 genera and 15 tribes of the Rubiaceae from South India are 1986 Pollen Nuclear Number in the Rubiaceae 123 reported. In general, the pollen nuclear number showed remarkable constancy within individual tribes and genera except the tribe Hedyotideae and the genus Oldenlandia. In great bulk of the species the vegetative nucleus underwent degeneration during the course of microspore development. Pollen nuclear number showed recognisable relationship with pollen aperture number, the binucleate species posses sing 3-aperturate grains and trinucleate ones six or more aperturate grains. The pollen nuclear number also showed an association with growth habit, the trinucle ate condition being present exclusively in herbaceous genera. These associations appear to be suggestive of a non-random distribution of the two nuclear types in the family. The systematic significance of pollen nuclear number in the family is discussed, supplemented with evidences from chromosomal and palynological data. As regards the delimitation and systematic position of various tribes, Bremekamp's classification is shown to be by and large in agreement with the cytological and palynological data. Affinities of the Rubiaceae with other families are examined in the light of the present data, and it is shown that the Gentianalean affinity is more natural.

Acknowledgements

We are deeply indebted to Prof. C. A. Ninan, Head of the Department of Botany, Kerala University for valuable suggestions, encouragements and for providing facilities for carrying out this work. One of us (OP) is thankful to the CSIR, Govern ment of India for the award of Research Fellowship during the period of this investi gation.

References

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