manuscript abstracts

Mon Jun 26 15:38:47 EST 1995

Martin, F.N. (1995) Linear Mitochondrial Genome Organization in
vivo in the genus Pythium.  Current Genetics 27:August issue

ABSTRACT - Pulsed field gel electrophoresis (PFGE) of isolates of
Pythium oligandrum with linear mitochondrial genomes revealed a
distinct band in ethidium bromide stained gels similar in size to
values estimated by restriction mapping of mitochondrial DNA
(mtDNA).  Southern analysis confirmed these bands were mtDNA and
indicated that linear genomes were present in unit length size as
well as multimers.  Isolates of this species with circular mtDNA
restriction maps also had low levels of linear mono- and multimers
visualized by Southern analysis of PFGE gels.  Examination of 17
additional species revealed similar results; three species had
distinct linear mtDNA bands in ethidium bromide stained gels while
the remainder had linear mono- and multimers in lower amounts
detected only by Southern analysis.  Sequence analysis of an
isolate of P. oligandrum with a primarily circular mitochondrial
genomic map and a low amount of linear molecules revealed that the
small unique region of the circular map (which corresponded to the
terminal region of linear genomes) was flanked by palindromic
intrastrand complimentary sequences separated by a unique 194 bp
sequence.  Sequences with similarity to ATPase9 coding regions from
other organisms were located adjacent to this region.  Sequences
with similarity to mitochondrial origins of replication and
autonomously replicating sequences also were located in this
region, their potential involvement in the generation of linear
molecules is discussed.

Martin, F.N. (1995) Electrophoretic Karyotype Polymorphisms in
Pythium species. Mycologia 87:333-353.

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 7 to 20  while summation of their sizes ranged
from 18.8 to 41.5 Mb.  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 S1
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.

A related manuscript on electrophoretic karyotypes was recently
published:  Martin, F.N. (1995) Meiotic instability of Pythium
sylvaticum as demonstrated by inheritance of nuclear markers and
karyotype analysis.  Genetics 139:1233-1246.

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