Cytology of Paspalum malacophyllum and its relationship to P-juergensii and P-dilatatum
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This study describes the meiotic chromosome pairing behavior and method of reproduction of 16 Paspalum malacophyllum accessions and their F1 hybrids with diploid P. juergensii (2n=2x=20) and tetraploid P. dilatatum (2n=4x=40). All P. malacophyllum accessions were tetraploids with 40 chromosomes. Their mean meiotic chromosome pairing was 0.82 I + 13.40 II + 0.02 III + 1.58 IV. As many as seven quadrivalents were observed, implying autotetraploidy. An examination of megasporogenesis and the subsequent gametophyte development revealed that all accessions were facultative apomicts with the form being apospory. All P. juergensii x P. malacophyllum hybrids were triploids with 30 chromosomes that associated primarily as 10 I + 10 II. The P. dilatatum x P. malacophyllum hybrids had 40 chromosomes that associated essentially as 20 I + 10 II. The 10 bivalents in both groups of hybrids represent autosyndetic pairing of the P. malacophyllum chromosomes, thus supporting autotetraploidy. The 10 univalents in the P. juergensii x P. malacophyllum hybrids are members of the J genome from P. juergensii (JJ), and the 20 univalents in the P. malacophyllum x P. dilatatum hybrids are members of the I and J genomes from P. dilatatum (IIJJ). Because P. malacophyllum does not possess the I or J genomes and its chromosomes pair autosyndetically, it was assigned the genome formula MMMM. Cytologically, both groups of F1 hybrids reproduced as facultative aposporous apomicts. In ovules with only sexual development, the female gametophyte frequently deteriorated prior to maturity. Several F2 progeny were recovered from both groups of F1 hybrids and had the same chromosome number as their meiotically unstable parental F1 hybrids. These F2 plants are the products of apomictic reproduction, demonstrating that apomixis is effective in perpetuating highly sterile, meiotically irregular interspecies hybrids.
