Cytological Studies on Three Aquatic Members of Hydrocharitaceae in Relation to Their Morpho-logical and Ecological Characteristics

Abstract

The present investigation has included the study of the morphological and cytological details of seventeen populations of three aquatic submerged taxa viz. Hydrilla verticilla Presl., Ottelia alismoides Pers. and Vallisneria spiralis L. belonging to the family Hydrocharitaceae. The species examined were collected from various localities in the plains of West Bengal.
The chief objective was to analyse the nature of association among different environmental factors, morphological variations and chromosomal peculiarities.
Morphological details, giving average leaf length and breadth and length of internodes, were observed in members of each population and the average calculated. Karyotype analysis was carried out on each population following pretreatmentorcein squash technique modified for the purpose. The pH and proportions of Ca, Mg, K and Na were calculated of the aquatic medium surrounding each population.
Eight populations of H. verticillata were studied. Seven show 2n=24 chromosomes while only one population has 2n=16 chromosomes. The karyotype formulae were observed to differ significantly.
The water analysis data indicated a wide range of variation in the proportions of minerals and of pH. No direct association among the morphology, water analysis data and the chromosome numbers was noted.
Four populations of Ottelia alismoides studied showed chromosome numbers 2n=44, 66 and 68. The populations with same chromosone numbers differ from each other in the structural composition of chromosomes. Population III (2n=68) is an aneuploid one which may have arisen through duplication of two chromosomes from a 2n=66 type or from 2n=33 type following duplication of the entire genome. The tetraploid populations I and II have a total chromatin length less than double those of III and IV indicating a diminution in chromosome size with polyploidy.
The water analysis data indicates that the species can tolerate wide chemical variations in their aquatic environment.
In the five populations of Vallisneria spiralis the chromosome number is 2n=30 for populations I and III, 2n=40 for II and V and 2n=24 chromosomes for IV. Karyotypes differ even among the populations with same number. Total chromatin length shows a proportionate increase during polyploidy.
Populations II and V with 2n=40 chromosomes have apparently evolved through polyploidy followed by structural changes in the chromosomes. Population IV represents a completely different picture. Its chromosome number may have been derived from 2n=40 or 2n=30 forms through non-disjunction and maintained through vegetative propagation.
There is no apparent association between the minerals present in the aquatic media and the chromosomal composition of the populations. The only exception is population IV.
The observations show that cytological races cannot be correlated with morphological characteristics. The water analysis data can suggest no specific association between the morphological and cytological characteristics, except that an alkaline medium of range of 6.8 to 8.55 is necessary.
In all the taxa studied vegetative mode of reproduction is an important factor of propagation. The occurrence of variant nuclei in the somatic cells indicates the way through which a new form could have arisen in nature. Such a variant nucleus, on entering a growing shoot, may give rise to a cytologically different form which may later form a colony under environmental conditions best suited for its survival. The excess of nutrients present in the surrounding media may permit all these forms to survive.
As far as the present study shows the alkalinity and mineral content do not have any definite effect on the evolution of cytological races. A possible role of the supply of nutrients through soil and of light intensity can be suggested.