SummaryPaspalum pumilum Nees had a chromosome number of 2n = 20. The chromosome number in P. arechavaletae Hack., P. cromyorhizon Trin. ex Doell., P. exaltatum F. & C. Presl, P. guenoarum Arech., and P. lividum Trin. ex Schlecht. was 2n = 40; and in P. mandiocanum Trin. it was 2n = 60. Meiosis was regular in P. pumilum, P. arechavaletae, P. exaltatum, and P. mandiocanum with primarily bivalent pairing. P. cromyorhizon and P. guenoarum had frequent quadrivalents, and meiosis in P. lividum was the most irregular with lagging chromosomes and micronuclei. A study of megasporogenesis and embryo sac development in each species showed that P. pumilum and P. arechavaletae were sexual and the other five species reproduced by apomixis. The type of apomixis was apospory and, with the exception of P. mandiocanum, species were pseudogamous.
Reciprocal hybridizations were made between yellow‐anthered dallisgrass, Paspalum dilatatum Poir (2n=4X=40), and four diploid species (2n=2X=20), P. intermedium Munro. ex Morong, P. jurgensii Hackel, P. notatum Flugge, and P. vaginatum Swartz. Percent cross‐ability ranged from 0 to 10.5. Generally a greater number of hybrids were produced with yellow‐anthered dallisgrass as the female parent. All hybrids were 2n=3X=30. The mean meiotic pairing relationships were: 10.14 I and 9.93 II for 63 yellow‐anthered P. dilatatum ✕ P. intermedium hybrids; 10.27 I and 9.86 II for one reciprocal hybrid; and 10.30 I and 9.85 II for 20 yellow‐anthered P. dilatatum ✕ P. jurgensii hybrids; and 10.38 I and 9.81 II for three reciprocal hybrids. Mean pairing relationships were: 27.96 I and 1.02 II for 28 yellow‐anthered P. dilatatum ✕ P. notatum hybrids; 28.56 I and 0.72 II for five reciprocal hybrids; and 28.07 I and 0.96 II for 38 yellow‐anthered P. dilatatum ✕ P. vaginatum hybrids. All hybrids were completely sterile. The P. intermedium and P. jurgensii genomes are homologous to those of yellow‐anthered P. dilatatum. These two diploid species are possible progenitors of yellow‐anthered P. dilatatum. Lack of chromosome pairing in the hybrids with P. notatum and P. vaginatum indicates they are not closely related to yellow‐anthered P. dilatatum.
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