Principles of Plant Genetics and Breeding

Figure 3 Range of seed development following pollination of
Poa arachnifera × P. secunda. Not all the hybrid seeds are viable
and they may require seed culture in nutritional media to
generate plants.
BREEDING CROSS-POLLINATED SPECIES 325
hybrids possessed a rhizomatous root system, which is
characteristic of the P. arachnifera parent, and suggests
that genetic control of the fibrous, bunchgrass
root system of P. secunda is recessive to the rhizomatous
root system of P. arachnifera.
Seed development
Stereoscopic observations regarding the progression of
seed maturity indicated a wide range in endosperm
and mature seed development. In some instances,
seed exhibited apparently normal endosperm development,
while other seed exhibited complete abortion
of the endosperm. Both the P. arachnifera and P.
secunda parents are highly polyploid (2n = 56 and 2n
= 63, respectively), which could cause their gametes,
following microsporogenesis and megasporogenesis,
to possess a wide range of chromosome numbers.
Ploidy differences in interspecific crosses are known
to account for endosperm developmental failure and
such events often lead to embryo abortion (Bradshaw
et al. 1995; Carputo 1997). It is assumed that the F 1
hybrids obtained in this study were obtained from seed
exhibiting a non-abortive endosperm.
Evaluations regarding the seed production potential
of the hybrids were determined by stereoscopic examination
following pollination and seed maturation. All
the hybrids were allowed to receive pollen through
open-pollination by adjacent pollen-fertile sibs or
nearby P. arachnifera males. For the dioecious females, self-pollination was impossible and pollen came from adjacent sibs or
parental material. A designation of sterile, low, medium, or high seed set was assigned to each monoecious or female dioecious
individual by general observations on the relative number of seed found throughout the seed heads. Of the 39 individuals possessing
female structures, 12 exhibited the capacity to set seed. Of the individuals setting seed, a wide range of viable or aborted
seed was observed (Figure 3).
Pollen fertility
Pollen fertility – as judged by pollen grain size, the occurrence of starch-filled pollen grains, and the presence of an operculum –
was found to vary among the hybrids. Upon examination of individuals representing both monoecious and dioecious hybrids,
pollen fertility as estimated by I 2 KI staining ranged from 0% to as high as 90%. Field observations estimated pollen viability in the
range of 0% to 80%. General comparisons of laboratory and field methods of estimating pollen viability in individual plants were
within 5–10% of each other.
To determine if the seed had been generated by either a sexual or an apomictic mode of reproduction, seed was sown and 10
seedlings were submitted to PCR genotyping utilizing a series of informative PCR-based markers. If following electrophoresis of
the PCR amplification products, band variation was observed among a set of siblings, sexual reproduction was indicated. If the
band profile was uniform across siblings and identical to the maternal parent, an apomictic form of reproduction was implicated.
Since no variation was observed, the analysis indicated that the seedlings were derived by an apomictic form of reproduction. As
a consequence, the parent from which these seedlings were derived likely produced its offspring via an apomictic form of reproduction.
Since P. secunda exhibits aposporic pseudogamous embryo sac development, this seed-fertile hybrid also likely
expresses this form of apomixis.
Conclusion
Though more detailed cytogenetic studies are necessary, the observed 10–90% pollen fertility scored in a random sampling of the
interspecific hybrids suggests that partial or high levels of genome affinity can exist between the P. arachnifera and P. secunda
genomes. The combining of these two genomes resulted in both pollen-fertile and seed-fertile F 1 hybrids, of which at least one
exhibited an apomictic form of reproduction. In addition, any heterotic response exhibited in the hybrids could potentially be
maintained via apomixis. The transfer of apomixis from P. secunda to the hybrids indicates that sexuality is recessive to apomixis.
Subsequent breeding of fertile interspecific hybrids and their utilization in a backcross program to P. arachnifera or P. secunda