Principles of Plant Genetics and Breeding

Potential pathways for apomixis gene introgression
PLANT REPRODUCTIVE SYSTEMS 65
Research strongly suggests that there is little homeology between the genomes of Tripsacum and maize. Maguire (1962) and
Galinat (1973), each utilizing a set of recessive phenotypic maize markers, suggested that only maize chromosomes 2, 5, 8, and 9
have potentials for pairing and recombination and for gene introgression with Tripsacum. Additional research has confirmed the
conservation of loci specific to pistil development between maize and Tripsacum genomes (Kindiger et al. 1995; Li et al. 1997).
Genomic in situ hybridization (GISH) studies have also strongly suggested that only three regions of maize chromosomes have
homeology with the Tripsacum genome: the subterminal regions of Mz2S, Mz6L, and Mz8L (Poggio et al. 1999). Though there is
little chromosome homeology, there is some hope for apomixis transfer from Tripsacum to maize. Two approaches that have
been successful in transferring components of apomixis from Tripsacum to maize can be detailed in two particular backcross
pathways (Harlan & de Wet 1977).
The first approach is called the 28 → 38 apomictic transfer pathway. This successful approach for apomixis transfer has been
described only once and has had little re-examination. In 1958, Dr M. Borovsky (from the Institute of Agriculture, Kishinev,
Moldova) performed a series of hybridizations between a diploid popcorn and a sexual diploid (2n = 2x = 36) T. dactyloides clone
with the first maize–Tripsacum hybrids being generated in 1960 (Borovsky 1966; Borovsky & Kovarsky 1967). The F 1 hybrids generated
from the experiments possessed 28 chromosomes (10Mz + 18Tr). The F 1 plants were completely male-sterile and were
highly seed-sterile. Backcrossing with diploid maize identified that some of the F 1 hybrids were approximately 1–1.5% seedfertile
and resulted in the production of progeny possessing 28 chromosomes (10Mz + 18Tr) and 38 chromosomes (20Mz + 18Tr).
When the F 1 was backcrossed to the Tripsacum parent, the fertile F 1 s generated progeny with 28 chromosomes (10Mz + 18Tr)
and 46 chromosomes (10Mz + 18Tr + 18Tr). The complete set of backcrosses with maize and Tripsacum resulted in a ratio of
approximately 10 (28-chromosome plants) to 1 (38- or 46-chromosome plants). Phenotypic observations suggested that the 28chromosome
progeny were not different from their 28-chromosome parent while the 38- and 46-chromosome progeny were
clearly different. Additional evaluations on the 28-chromosome F 1 and its 28-chromosome progeny suggested that these F 1 plants
and their progeny were apomictic. This early experiment remains the single incidence where a 28-chromosome F 1 hybrid was
maintained by apomixis.
A second pathway whereby apomixis has been introgressed from Tripsacum to maize is the 46 → 56 → 38 apomictic transfer
pathway. Though not specifically addressed in the definitive work on maize–Tripsacum introgression (Harlan & de Wet 1977),
this successful attempt at apomixis transfer requires a brief reiteration. Initially published by Petrov and colleagues as early as
1979, and replicated in similar style by others, a diploid or tetraploid maize line is pollinated by a tetraploid, apomictic T. dactyloides
clone (Petrov et al. 1979, 1984). If a diploid maize line is utilized, the resultant F 1 46-chromosome hybrid possesses 10Mz
and 36Tr chromosomes. Upon backcrossing with diploid maize, both apomictic 46-chromosome and 56-chromosome (20Mz +
36Tr) individuals can be obtained. The 46-chromosome offspring are products of apomixis. The 56-chromosome offspring are
products of an unreduced egg being fertilized by the diploid maize pollen source, another 2n + n mating event. Backcrossing the
46-chromosome individuals by maize, repeats the above cycle. Upon backcrossing the 56-chromsome individuals with maize,
three types of progeny can be observed. Typically, progeny having 56 chromosomes are generated. However, in some instances,
2n + n matings occur giving rise to individuals possessing 66 chromosomes (30Mz + 36Tr). Occasionally, a reduced egg will be
generated and may or may not be fertilized by the available maize pollen. In rare instances of non-fertilization, a 28-chromosome
individual is generated (10Mz + 18Tr). In instances whereby the maize pollen fertilizes the reduced egg, 38-chromosome individuals
are obtained (20Mz + 18Tr). Generally, individuals possessing 38 chromosomes, rather than 28 chromosomes, are the
most common product. What is unique about this pathway is that, occasionally, the 38-chromosome individuals retain all the
elements of apomixis that were present in the Tripsacum paternal parent and the F 1 and BC 1 individuals (Figure 1). The retention
of apomixis to this 38-chromosome level has been well documented and repeated in several laboratories (Petrov et al. 1979,
1984; Leblanc et al. 1996; Kindiger & Sokolov 1997).
Generally, through 2n + n mating events, the 38-chromosome individuals produce only apomictic 38-chromosome progeny
and 48-chromsome progeny. Backcrossing the 48-chromsome individuals results in 48-chromosome apomictics and 58chromosome
apomictics. Each of these steps gives rise to a different plant and ear phenotype (Figure 2). This 2n + n accumulation
of maize genomes continues until a point is achieved where the additional maize genomes eventually shift the individual
from an apomictic to a sexual mode of reproduction. It is extremely difficult to generate and maintain apomixis in hybrids possessing
fewer than 18Tr chromosomes. Likely this is due to the expression of apomixis in the 38-chromosome hybrids. However,
in one instance an apomictic individual possessing 9Tr chromosomes has been identified (Kindiger et al. 1996b) suggesting that
with time and patience, additional Tripsacum chromosomes can be removed from these hybrids and still retain the apomixis.
Recent attempts to transfer apomixis from Tripsacum to maize
As of this report, prevailing wisdom suggests that apomixis (at least for Tripsacum) is controlled by no more than one or two genes,
likely linked on a particular Tripsacum chromosome (Leblanc et al. 1995; Grimanelli et al. 1998). Cytogenetic and GISH studies
suggest this region may be Tr16L in the vicinity of the nucleolus-organizing region that has homeology with the distal region of
Mz6L (Kindiger et al. 1996a; Poggio et al. 1999). Evaluations of materials from the Petrov program and generated through the
46 → 56 → 38 pathway have identified an apomictic line that does not possess an intact Tr16 chromosome or the Mz6L–Tr16L