Hybridization was attempted between Zea mays and Tripsacum dactyloides with the purpose of studying chromosome affinity between both species and the possibility of introgressing some characters from the wild species to maize. Crossings between Z. mays (2n=20-40) and T. dactyloides (2n=72) showed post-zygotic incompatibility. Between 12 to 17 days after pollination, embryos aborted because of endosperm collapse. Reciprocal crosses induced parthenogenesis in T. dactyloides.
Embryo rescue and induction of organogenesis and somatic embryogenesis allowed us to obtain 5 hybrid plants with 2n=46 (ZT46) and 188 plants with 2n=56 (ZT56), from crosses between Zea mays 2n=20 (Zm20) x T. dactyloides and Z. mays 2n=40 (Zm40) x T. dactyloides, respectively (Fig. 1a and b).
The most frequent meiotic configurations in parents and hybrids were:
Zm20 = 10 II
Zm40 = 10 IV or 9 IV + 2 II
Td = 26 II + 5 IV or 24 II + 6 IV.
ZT46 = 18 II + 10 I or 16 II + 14 I.
ZT56 = 28 II or 26 II + 4 I.
In the hybrid ZT46, homoeologous chromosomes of T. dactyloides paired amongst them whilst those of Zm20 remained as univalents. Pairing between the chromosomes of both species was exceptionally observed, building one or two trivalents. Anaphase was not regular, with lagging chromosomes and a different chromosome number at each pole. Pollen fertility was determined with Lugol and ranged from 0 to 60%. No viable seeds have been obtained to date.
In the hybrid ZT56, homoeologous chromosomes of T. dactyloides and Zm40 homologous ones paired amongst themselves, with the most frequent meiotic configurations 28 II (Fig. 2a) or 26 II + 4 I. The average of meiotic configurations in ZT56 was 0.61 I + 25.16 II + 1.29 IV. In some cases, tetravalents were the result of chromosome pairing between both species (Fig. 2b). Exceptionally, chromosomes paired into hexavalents (average/cell = 0,02%) or they stuck, building multivalents (Fig. 2c).
80% of anaphases were regular and in the remaining 20% lagging chromosomes were observed (Fig 2d) as well as inversion bridges. The pollen was completely sterile. Viable seed was obtained from the 50% of the plants pollinated with Zm20, Zm40, T. dactyloides or Z. perennis. The plants originating from these seeds had a chromosome number 2n=56. Apparently they originated by apomixis.
Figure 1. Karyotypes of the hybrids between Zm20, Zm40 and T. dactyloides. a- ZT26 (2n=46), b- ZT56 (2n=56).
2. Meiotic configuration of the hybrids ZT56. a- 28 II; b- 26 II +
1 IV; c- sticking chromosomes; d- Anaphase with set lagging chromosomes.
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