s - Mycological Society of America

elationships among taxa at the species level
and beyond. Techniques such as PCR enable
the user to examine older herbarium specimens
to determine relationships with living
populations. Other studies examine groups of
taxa by classical morphological methods
combined with genetic and molecular studies.
Voucher specimens must be annotated in
herbaria and so should vital information
derived from the genetic an molecular
studies. Many questions have arisen
concerning the preservation of DNA and RNA,
storage of such material, annotation of
studied material, use of old and new
holotypes, etc. At present little
information exists on the quantities of
material needed for various studies, the
.preservation of copies or photos of gels, DNA
"sequences and how to indicate exactly what
material in a given collection was sampled.
The round table discussion will attempt to
examine the present and future problems and
the issues which generate the greatest
concerns for researchers and curators alike.
The objective will be to draft a curatorial
policy and uniform approach which can be
presented to the MSA members and eventually
implemented as policy by curators of
mycological collections for the greatest
benefit to science.
C. U. M1Mlr1, E. A. RICHARDSON' and J. KIMBROUGH~.
l~e~artment of Plant Pathology. University of Georgia,
Athens, GA 30602 and *Department of Plant Pathology,
University of Florida, Gainesville. FL 32611.
Ultrastructure of ascospore delimitation in freeze
substituted samples of as codes mi^ hricans.
Freeze substitution proved to be a useful technique
for studying the early stages of ascosporogenesis in
Ascodesnis niericang. Our observations indicate that
the ascus vesicle originated from the ascus plasma
membrane. Invaginations of the plasma membrane
produced ascus vesicle initials consisting of two
closely spaced unit membranes. The appearance of the
outer leaflet of each of these membranes was identical
to that of the inner leaflet of the ascus plasma
membrane. Apparent points of continuity between ascus
vesicle initials and the plasma membrane were
observed. Ascus vesicle initials accumulated in the
ascus cytoplasm near the plasma membrane and then
coalesced to form the ascus vesicle, a peripheral,
cylinder-like structure consisting of two closely
spaced unit membranes that extended from the ascus
apex KO the ascus base. The ascus vesicle then became
invaginated in a number of regions and subsequently
gave rise to eight sheet-like segments, or ascospore-
delimiting membranes. that encircled uninucleate
segments of cytoplasm forming ascospore initials.
Like the ascus vesicle, each ascospore-delimiting
membrane consisted of two closely spaced unit
membranes, the inner of which became the ascospore
plasma membrane. The ascospore wall developed between
the spore plasma membrane and the outer membrane.
Many details of ascospore maturation were clearly
visible in freeze substituted samples.
C. W. HlMS and K. M. SKETSELAAK. 1)epartment of Plant
pathology, Univ. of Georgia. Athens, CA 30602.
An ultrastructural study of teliospore maturation in
the smut fungus Sporisoriwn sorghi using freeze
substitution fixation.
l'eliospores of the smut Sporisorium sorghi Lanpdon and
Fullerton developed in galls produced in Sorghum
Iralepense inflorescences. Small pieces of galls were
freeze substituted and processed for study with TEM.
This procedure yielded well-preserved spores in
various staRes of maturation, and permitted detailed
u?trastructural observations of stages difficult to
preserve with conventional fixation pethods.
Walls of sporogenous hyphae gelatinized, leaving
uninucleate and apparently wall-less spore initials.
Young teliospores then became surrounded by an
electron.transparent primary wall. Electron dense,
spine-like spore surface ornamentations developed
adjacent to the plasma membrane and grew into the
primary wall, which persisted as a sheath around the
enlarging spines. A uniform layer of electron dense
wall material was subsequently deposited beneath the
spines. As spores matured, a less electron dense.
fibrillar inner wall layer developed.
Our interpretation of early stages of teliospore wall
development is consistent with light microscope
observations of S. sorghi and related species which
describe gelatinization of sporogenous hyphal walls
and development of spores frolo naked protoplasts. It
differs from descriptions of taliosporogenesis in
Tilletia species, where the primary spore wall does
not arise de novo bur is continuous with the wall of
the sporogenous hypha.
P. L. MINEHART and B. MAGASANIK. Department of
Biology, Hassachusetts Institute of Technology,
Cambridge, MA 02139.
Regulation of nitrogen assimilation.
In 2. cerevisiae, at least two Independent systems
exist which regulate the expression of genes involved
in the assimilation of nitrogen. The first system,
which responds to the intracellular glutamine to
gltuamate ratio, regulates several genes including
GLNl (glutamine synthetase), CDH2 (NAD-linked glutamate
dehydrogenaae), and (general amino acid
permease). In wild-type cells, the transcription of
these genes is repressed on glutamine and derepressed
on glutamate. Two genes involved in this regulatory
pathway. URE2 and E, have been defined. URE2 encodes
a negative regulator which is believed to control
the product of =, a positive regulator which
contains a putative zinc finger DNA binding domain.
Upstream analysis of various genes under the
--
URE2/GLN3 control has identified a consensus se-
quence required for this control.
The second system, which is less well defined,
regulates protein levels in response to the presence
or absence of a nitrogen source. The genes for
amino acid permeases, including w, are subject to
this regulation, which works at the transcriptional
level. These permeases can be further subjected to
post-transcriptional inactivation by either ammonia
or glutamine.