1022-7954/05/4104- © 2005 Pleiades Publishing, Inc.
Russian Journal of Genetics, Vol. 41, No. 4, 2005, pp. 402–411. Translated from Genetika, Vol. 41, No. 4, 2005, pp. 508–519.
Original Russian Text Copyright © 2005 by Zakharov.
Reproduction of most eukaryotic organisms
involved alternation of ploidy (haploid and diploid
phases), which is provided by alternation of two pro-
cesses: fusion of nuclei and meiosis. The fused nuclei
may be of different origin: they may be produced by
different, related or unrelated, individuals or by one
individual. Nuclear fusion types are classiﬁed  as
allogamy (fusion of nuclei originating from different
zygotes), autogamy (fusion of nuclei derived from one
zygote but formed in different meiotic divisions), and
automixis. Automixis is the appearance of a new indi-
vidual originating from the product(s) of one meiotic
division. The meiotic product may be a haploid nucleus
or a cell containing such nucleus. Thus, intratetrad mat-
ing discussed in this article, is one of the forms of auto-
SEXUAL REPRODUCTION, AUTOMIXIS,
AND INTRATETRAD MATING
which is widely used
in biology, does not have a strict deﬁnition . Differ-
ent authors consider the same phenomenon as sexual or
asexual reproduction. Mogie  coined the following
working deﬁnition: “sexual reproduction is any process
of appearance of a new individual involving fusion of
nuclei irrespective of their origin and ploidy, and the
genetic status of the resultant genotype (i.e., its homo-
If we accept this deﬁnition, then automixis is a
mixed notion referring to two different phenomena:
(i) appearance of a new individual that does not
involve nuclear fusion: endomitosis either precedes or
follows meiosis, resulting in the formation of a diploid
nucleus that gives rise to a new organism;
(ii) intratetrad mating: fusion of the haploid nuclei
within a tetrad, i.e., direct products of a meiotic division
or their mitotic progeny.
According to the above deﬁnition, only the latter
form of automixis can be regarded as a sexual process.
In literature (see  for review), different opinions are
voiced as to whether apomixis should be regarded as a
sexual process or included in the mechanisms of repro-
duction known as parthenogenetic.
In conclusion, note that automixis in form of
intratetrad mating is rather widely spread among fungi
and occurs in a number of parthenogenetic insects.
Recently, intratetrad mating has received increasing
attention, since it is an obligatory or facultative element
of the life cycle of commercially cultivated or model
species of fungi.
INTRATETRAD MATING IN FUNGI
AS A SPECIAL FORM OF MATING
BETWEEN CLOSE RELATIVES
In many fungi and algae, the products of individual
meiosis are linked until their maturation, forming a tet-
rad of spores (gametes). An additional mitotic division
after meiosis produces a spore octad. Because of the
Intratetrad Mating and Its Genetic
and Evolutionary Consequences
I. A. Zakharov
Vavilov Institute of General Genetics, Russian Academy of Sciences, Moscow, 1199991 Russia;
fax: 135-89-62; e-mail: email@example.com
Received December 15, 2004
—Genetic characteristics of intratetrad mating, i.e., fusion of haploid products of one meiotic division,
are considered. Upon intratetrad mating, the probability of homozygotization is lower than that upon self-fer-
tilization, while heterozygosity at genes linked to the mating-type locus, which determines the possibility of
cell fusion, is preserved. If the mating-type locus is linked to the centromere, the genome regions adjoining the
centromeres of all chromosomes remain heterozygous. Intratetrad mating is characteristic of a number of fungi
Saccharomyces cerevisiae, Saccharomycodes ludwigii, Neurospora tetrasperma, Agaricus bisporus, Micro-
, and others). Parthenogenetic reproduction in some insects also involves this type of fusion
of nuclei. Intratetrad mating leads to the accumulation of haplolethals (i.e., lethals manifesting in haploid cells
but not hindering their mating) in pericentric chromosome regions. Since heterozygosity increases viability of
an organism, recombination has been suppressed during evolution in fungi characterized by intratetrad mating,
which ensures heterozygosity of the most part of the genome.