Gesneriaceae:
Chromosome Numbers
 

 

At present, no basic number can be identified for the family as a whole. In general, the basic numbers are moderately high, ranging between n = 8 and 17. The lowest number found so far is n = 4 (Chirita pumila), the highest is n = 64 (Streptocarpus hildebrandii). In the Gesnerioid Gesneriaceae the chromosome numbers are fairly constant as to genera and tribes, while they are very variable in the other groups.

 

Coronatheroid Gesneriaceae

In this small group generally high numbers are found (Mitraria, Sarmienta, Rhabdothamnus: n = 37, Fieldia: n = 40, Negria: n = 45), apparently due to (paleo)polyploidy. 

 

Gesnerioid Gesneriaceae

In this group chromosome numbers are largely consistent with genera. If dysploidy is taken into account, the chromosome numbers are even consistent within tribes: in Gloxinieae most genera have x = n = 13, a few n = 12, 11 or 10, in Episcieae the number is n = 9 almost throughout (two genera have n = 8), in Sinningieae n = 13, in Gesnerieae n = 14. Based on a combination of molecular and karyological data two base numbers are assumed for neotropical Gesneriaceae: n = 16 (Beslerieae and Napeantheae) and n = 13 (Gloxinieae and Sinningieae, increase to n = 14 in Gesnerieae) and reduction to n = 9 in Episcieae. (with occasional reduction to n = 8).

Polyploidy is remarkably rare in the neotropical Gesneriaceae. Even large genera such as Columnea, Sinningia, Nautilocalyx etc. are highly uniform and contain at most a few tetraploid species.

 

Epithematoid Gesneriaceae

This group is heterogenous with regard to chromosome numbers, not only as to the genera, but also as to the species within some genera. The lowest number is n = 8 (Whytockia, Epithema p.p.), the highest is n = 27 (Rhynchoglossum papuae, R. omeiense). In Monophyllaea at least three numbers are found (n = 10, 11, 12), in Rhynchoglossum even four (n = 10, 18, 21, 27).

 

Didymocarpoid Gesneriaceae

For this group fewer data are available than for the and equally large Gesnerioid Gesneriaceae, and the evolutionary diversification seems to be more complicated. The largest genus, Cyrtandra, with perhaps 600 species, is remarkably uniform (n = 17, the few reported exceptions are doubtful). Other genera with a single number include Agalmyla (n = 16), Ridleyandra (n = 17) and Petrocosmea (n = 17), but relatively few species have been counted in these genera.

In a number of genera two or several basic numbers are found in one genus, evidently due to ascending or descending dysploidy (e.g., Streptocarpus: n = 15, 16; Aeschynanthus: n = 14, 15, 16; Didymocarpus: n = 11, 12, 14, 16, 18), Monophyllaea (n = 10, 11, 12).

Polyploidy occurs in a fair number of Old World Gesneriaceae and apparently has significantly contributed to speciation. In the large African and Madagascan genus Streptocarpus (c. 135 spp., basic numbers x = 15 and 16) polyploid species (tetra- and octoploid, n = 30, 32, 48, 64) are only found in Madagascar and the Comoro Islands, while the species on the African mainland remained on the diploid level. Polyploidisation has also played a major role in the evolutionary diversification of Aeschynanthus (c. 160 spp., n = 14, 15, 16). In this genus polyploidy (tetra- to octoploidy) is not only characteristic of particular species, but different ploidy levels even occur within individual species, e.g., A. parvifolius and A. ellipicus. In the latter species even three different ploidy levels (2x, 4x, 6x) have been reported. As far as known, in these species the diploids and polyploids cannot be distinguished morphologically, and polyploidisation may be the first step of speciation. The great variety of chromosome numbers found in Chirita (n = 4, 9, 10, 14, 16, 17, 18) is presumably due to the artificial nature of the genus and cannot be simply interpreted in terms of dysploidy and polyploidy.

 

Chromosome size

In general, the chromosomes of Gesneriaceae are very small. They measure usually less than 1 mm in mitotic metaphase. Larger chromosomes have been found only in Agalmyla, Monophyllaea and Whytockia, measuring up to 3 mm.

 

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