Cytogenetics of Spartina alterniflora

The chromosome numbers of S. alterniflora existed a high degree of polyploidy with chromosomes present in multiples of seven (Church, 1940). Octoploids with 56 and the decaploids with 70 chromosomes have been reported, and two levels of polyploidy within S. alterniflora were found to be morphologically and ecologically distinct. Thus, Church (1940) concluded that the basic number of chromosomes within this genus is seven. However, controversy existed as to whether the tall and short forms of S. alterniflora differ genetically or simply reflect differences in adaptation to the environment to which they are exposed. The short form, S. alterniflora var. glabra (Muhl.), was characterized as being octoploid (n=7) with 56 chromosomes and the tall form, S. alterniflora, var. pilosa (Merr.), was characterized by being a more robust decaploid (n=7) with 70 chromosomes. A cytological study of S. alterniflora populations in northeastern U.S.A. and Canada conducted by Marchant (1970), excluded S. alterniflora from the basic number of 10 and affirmed that both the short and tall forms have the same chromosome number of 62. This is unusual with respect to the basic number of x=10 generally found in the genus. It is not known where the two additional chromosomes in the complement of S. alterniflora originated. Compared with the basic number observed in other species, it is possible that they represent a tetrasomic condition.

2.4 Reproductive Biology of Spartina Alterniflora

Flowers of S. alterniflora are smooth, lack hairs, and panicles are erect to arching. The panicles are usually 10 to 45 cm long with 5 to 30 spikes alternately arranged and appraised to main axis with 10 to 50 sessile spikelets along one side of the axis of each spike. Under favorable conditions, S. alterniflora can reach sexual maturity in three to four months (Smart & Barko, 1982). S. alterniflora responds to short day length for plant growth and flowering (Seneca and Broome,1972). Flowering occurs earlier under short-day than under long-day conditions in 22-26°C and 26-30°C temperature conditions. Also, Seneca (1974) found considerable morphological and physiological variation among populations of S. alterniflora from the Atlantic and Gulf coasts. Whereas southern populations flowered later and exhibited a longer growing period, they showed less sensitivity to changes in photoperiod than northern populations.

S. alterniflora flowers annually with variable flowering dates throughout its geographic distribution (Mobberley, 1956), and the species is protogynous, meaning that female flowers mature before male flowers, and this strategy helps ensure outcrossing. Stigma protruded from the floret 2-5 d before anther and pollen germinated about 15 min after contacting the stigma, and pollen tubes grew to the micropyle within 55 to 75 min (Fang et al., 2004). Daehler and Strong (1994) reported that inflorescences of self-pollinated plants generally had lower seed set than that of controls, and none of the self-pollinated seed were viable. Spikelet viability was not a function of clone size but there was some genetic control of spikelet viability shown by the uniformity of seed set that existed within large clones, and there was inbreeding depression with different degree among clones, which may be due to the S. alterniflora populations on the west coast were established from a relatively small number of genetic individuals (Daehler and Strong, 1994).

According to Mooring et al. (1971), seeds are set in fall and subsequently dispersed but remain dormant until early the next spring. The phenomenon of dormancy can help seed survive through a cold winter. Seed output of both native and introduced Spartina populations has been found to be highly variable (Callway and Josselyn, 1992). Seed production in S. alterniflora marshes has also been found to be unpredictable across years (Broome et al., 1974), and the plant does not produce seed in several areas where it has been introduced. No flowers have been observed in New Zealand or in Padilla Bay (Pacific Northwest coast of the U.S) populations for almost 50 years after its introduction (Partridge, 1987). Low soil temperature can delay or suppress flowering and reduce seed production in Spartina. Callaway and Josselyn (1992) reported that seed production was very erratic as evidenced by the variation in the number of seeds produced per flower, ranging from 3% to 20% during a two year period of evaluation. The germination rates ranged from 0% to 59% among clones (Daehler and Strong, 1994). Variable germination rates were also observed by Callaway and Josselyn (1992) with roughly 37% of seeds collected in San Francisco Bay. Sayce (1988) found only a 0.04% germination rate for seeds collected in Willapa Bay. However, Qin et al. (1985) reported that the germination rate of seeds of Florida form (F-form), which introduced from Florida state, USA, was over 90% with distilled water, and that of other two ecotypes, Georgia form (G-form) and North Carolina form (N-form) which introduced from Georgia state and North Carolina state respectively, were 76% and 66%. In addition, there were significant differences among the germination potential of three ecotypes. The results indicated that the germination characteristics of S. alterniflora seeds varied with the sources of seeds.

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