RE: BRYONET: local endemics

BRYONET

Ah, too late, Jon. Apparently the topic has not been amply discussed,
witness the continued discussion.

Anent this, something I'd like to bring up is the chromosome number
variation within species, and its significance or lack thereof in modern
systematics. Remember chromosome numbers, Bryonetters?

An examination of Fritsch, R. 1982. Index to Plant Chromosome
Numbers-Bryophyta. Bohn, Scheltema & Holkema, Utrecht & Antwerp shows
many species with only one haploid chromosome number, but also
demonstrates significant variation of haploid chromosome numbers (many
derived, however, from counting meiotic figures) within many species of
various bryophyte families. These are usually aneuploid, but also
polyploid.

Examples from Pottiaceae:
Barbula unguiculata usually n = 13 or 13+m, but also 14, 14+2m, 12+m,
11+m.
Didymodon rigidulus and D. tophaceus, usually n = 13, but also 12.
Hymenostomum microstomum, usually n = 13, but also 26.
Hyophila involuta, usually n = 13, but also n = 7.
Pottia bryoides, n = 15 or 52.
Pottia lanceolata, usually n = 26, but also 13 and 24.
Pottia truncata, usually n = 26, but also 20, 25, and 52.
Tortella humilis n = 15 or 26.
Tortella calycina n = 13, 30 or 52.
Tortula desertorum usually n = 12, but also 3.
Tortula laevipila n = 12, 15 or 26.
Tortula muralis (copiously counted) n = variously 13+m, 24, 26, 27, 40,
48, 50, 52, 60 and 66.
Tortula princeps n = 12, 24, 24+m, 26, 28, 36+2m.
Tortula robusta n = 7 or 12.
Tortula ruralis n = 12 or 26.
Tortula subulata n = 14, 24, 26, 48.

In other families, a similar aneuploid or polyploid variation obtains.
For instance, Funaria hygrometrica usually has n = 14 or 28, but also
56.

None of these species are difficult to identify, and most have a
distinctive microhabitat that may provide stabilizing selection on the
phenotype for that particular expressed structure and its phyletic
constraint, and any characteristic geographic ranges are difficult to
infer.

Given that all these intraspecific chromosomal variants doubtless are to
a large extent genetically isolated, shall we split each species such
that the variants are cryptic species? Clearly not, and this has been
the verdict of history regarding chromosome number variation. This is
another example of punctuational (initial change followed by stasis)
evolution of expressed traits, and gradualist variation of those traits
of the genome that are nearly neutral.

*****************************
Richard H. Zander
Voice: 314-577-0276
Missouri Botanical Garden
PO Box 299
St. Louis, MO 63166-0299 USA
richard.zander@mobot.org
Web sites: http://www.mobot.org/plantscience/resbot/
and http://www.mobot.org/plantscience/bfna/bfnamenu.htm
Non-post deliveries to:
Missouri Botanical Garden, 4344 Shaw Blvd., St. Louis, MO 63110
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