Title
Electrophoretic Karyotype Polymorphisms in the Genus Pythium
Abstract
Inter- and intraspecific variation of the electrophoretic
karyotypes of 18 Pythium spp. were evaluated
by pulsed field gel electrophoresis. All species
differed in karyotype, and while some morphologically
similar species shared common ranges in chromosome
size, this was not a consistent characteristic. Numbers
of chromosomal bands for individual isolates ranged
from seven to 20, while summation of their sizes ranged
from 18.8 to 41.5 megabases. The extent of intraspecific
variation was determined by examining 75 geographically
diverse isolates representing 10 species.
While there was general conservation in the size range
of the chromosomes within a species, significant levels
of intraspecific polymorphisms were identified. These
differences were observed as variation in the number
of chromosomal bands as well as summation of their
sizes and differed by as much as 40% within a species.
For most species differences in summation of chromosome
sizes could be attributed to variation in the
numbers of chromosomes present; however, for several
species this also could be attributed to differences
in the size distribution of the chromosomes. Depending
on the species, rDNA was encoded on from one
to five chromosomes, a feature that was generally conserved
within a species. Karyotypes of P. myriotylum
appeared to be mitotically stable with no apparent
variation observed following repeated mycelial transfers
over a period of a year. Meiotic stability of karyotypes
was evaluated by selfing several homothallic
species. For P. oligandrum, only one out of 28 SI progeny
differed from the parent and this was due to the
loss of a putative supernumerary chromosome. In contrast,
while all 28 selfed progeny of P. spinosum were
nearly identical to the parental karyotype, 61% differed
in size or presence of at least one chromosomal
band. Possible mechanisms responsible for this meiotic
instability of P. spinosum and the observed level
of intraspecific variation in the genus are discussed.
In view of the levels of intraspecific polymorphism and
meiotic instability of some isolates, analysis of electrophoretic
karyotypes will have limited taxonomic utility.
Authors
Frank N. Martin
Journal
Mycologia, 1995 : 87(0), 333~353
