Cytology of Nepenthes
Nepenthaceae are remarkably uniform in chromosome number with 2n=80. There is good evidence that x=5 (x=10) is the basic chromosome number for the family.

Chromosome size, condensing behaviour of chromatin and interphase nuclei are very similar to that of Droseraceae.

Chromosome numbers of selected species of Nepenthes


Investigated species: all 2n=80

Nepenthes madagascariensis, N. pervillei, N. distillatoria, N. khasiana, N. rafflesiana, N. truncata, N. stenophylla, N. gracilis, N. eymai, N. thorelii, N. veeitchii, N. albomarginata, N. reinwardtiana, N. tentaculata
Somatic, mid-metaphase chromosomes of 6 species of Nepenthes

Somatic, mid-metaphase chromosomes of 6 species of Nepenthes:

A - N. madagascariensis    B - N. pervillei    C - N. clipeata
D - N. rafflesiana    E - N. distillaroria    F - N. albomarginata

A cytological comparison Nepenthaceae - Droseraceae

Recent molecular studies and cladistic analysis of rbcL data (ALBERT et al. 1992; CHASE et al. 1993) indicate that Nepenthaceae may be surprisingly closely allied to Droseraceae and also to the Caryophyllales.

These affinities of Nepenthaceae and Droseraceae are strongly supported by the cytological data. Both families share common karyomorphological features.

All chromosomes are generally very small in size and only in Droseraceae a classification in small, medium andflarge chromosomes (or karyotypes) is possible.

The condensing behaviour, the chromatin and interphase nuclei structure is also very similar.

In contrast to Droseraceae which probably lack a primary constrictions (KONDO & LAVARACK, 1984) centromeres have been observed in the larger chromosome of Nepenthes.

In Droseraceae the most frequent basic number appears to be x=5 (x=10), being found in many taxa especially in Drosera subg. Drosera.

In other sections of Drosera various chromosome numbers have been reported (KONDO 1966; 1969; 1970; 1971; 1973; 1976; 1984) with primary basic numbers x=6,7,8,9 and perhaps also secondary basic numbers x=11 and x=13.

Aneuploidy seems very common especially in some Australian groups of Drosera.

For the related genera Dionaea and Aldrovanda x=8 has been established and for the genus Drosophyllum the basic number x=6 is accepted.

It is remarkable that Droseraceae as well as Nepenthacae have reached the same ploidy level (8x or 16x). Available data suggest x=5 or x=10 may be ancestral for both families.

All available data show that Droseraceae exhibit an enormous spectrum of chromosome numbers.

Hybridisation, parallel polyploidisation on different base numbers an numerical changes were probably involved in the evolutionary process.

It is evident that in Droseraceae all ploidy levels are present, whereas in Nepenthes diploid and lower polyploids have already gone extinct and the radiation was restricted to a high ploidy level.

In view of the phylogenetic background one can assume that Nepenthaceae are palaeopolyploids with the basic number x=20.

The loss of taxa with lower ploidy levels, the high chromosome number, the isolated taxonomic position, the palaeotropic distribution, the uniformity in many characters and the reduced genetic variability support this assumption.

On the other hand, fossil record, the specialized morphology and molecular data are not in favour of this hypothesis.

In view of the molecular data (rbcL), palynological (tetrads), chemical (naphthochinons) and morphological features (fruit, seeds, embryo, ovules) and also from the cytological point of view a systematic position of the Nepenthaceae not to far away from the Droseraceae is confirmed.

Summary of chromosome numbers, ploidy levels and basic numbers ascertained
for Nepenthaceae and Droseraceae.
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