Unreduced gametes and neopolyploids in natural populations of Achillea borealis (Asteraceae)

Abstract

Polyploidy is a major mechanism of speciation and adaptation, yet little is known about the origins of polyploids in natural species. I investigated gametic nonreduction and neopolyploid formation in natural tetraploid populations of Achillea borealis (Asteraceae), an autopolyploid complex consisting of tetraploid and hexaploid cytotypes. Cytological analyses of tetraploid populations revealed the occurrence of reduced (n ¼ 2x) as well as unreduced 'big' (2n ¼ 4x) and 'jumbo' (4n ¼ 8x) pollen grains, which were clearly distinguished by size. Production of unreduced pollen was monitored in two tetraploid populations in 1997 and 1998. Mean population-level frequencies of unreduced pollen ranged from 0.030 to 0.538%, with as few as one-third and as many as one-half of sampled plants producing unreduced grains. Eight individuals were found to produce 41% unreduced pollen, with highest observed frequencies of 7.0, 13.2 and 15.8%. Experimental crosses using high unreduced pollen producers as male parents generated viable seeds. However, the frequency of neohexaploids in the progeny of experimental crosses (0.388%) was similar to that observed in progeny of randomly selected, openpollinated control parents (0.465%). These results suggest that unreduced eggs are the most likely source of new polyploids. In spite of the inefficiency of unreduced pollen in unilateral sexual polyploidization, the overall rate of neohexaploid formation (one in 233) was several orders of magnitude greater than estimates of genic mutation rates.

Genetic and environmental determinants of unreduced gamete production in Brassica napus, Sinapis arvensis and their hybrids

“…The 0.06 to 2.17% range is consistent with the observation of Ramsey (2007) that 0.1 to 2% of gametes are expected to be unreduced in nonhybrid populations. As well, the skewed distribution of unreduced gametes, with most individuals having low rates and a few with much higher rates, has been observed before when unreduced gametes have been quantified across large numbers of individuals (Ramsey, 2007;Dewitte et al, 2012).…”

“…The 0.06 to 2.17% range is consistent with the observation of Ramsey (2007) that 0.1 to 2% of gametes are expected to be unreduced in nonhybrid populations. As well, the skewed distribution of unreduced gametes, with most individuals having low rates and a few with much higher rates, has been observed before when unreduced gametes have been quantified across large numbers of individuals (Ramsey, 2007;Dewitte et al, 2012). The combination of low-level baseline unreduced gamete production along with a single high-level producer highlights the variable nature of unreduced gamete production that may be a product of environment, development and genetics (Ramsey and Schemske, 1998).…”

“…The best-fit line (mean diameter = 28.0+47.5 u) predicts a mean diameter for pollen with reduced gametes that was consistent with our microscopic examinations (mean diameter = 28.0 μm when u = 0), but a diameter for pollen with unreduced gametes of 75.5 μm (when u = 1). Not only does this not correspond to the expectation that pollen with unreduced gametes should be less than twice the diameter of pollen with reduced gametes (Ramsey, 2007), it also would mean that in samples with unreduced gametes, the larger pollen would not be within the size range we measured (20 to 40 μm). The observed correlation therefore must be explained in some alternative way.…”

“…Traditionally, pollen size and microspore analysis have been used to measure the frequency of pollen with unreduced gametes (hereafter referred to as 'unreduced pollen'). Estimates based on pollen size rely on finding oversized pollen grains, typically 30-40% larger than reduced pollen (Ramsey, 2007). This technique is limited by the number of pollen grains that can be visualized and measured when done microscopically, although this limitation is overcome when volumetric measurement is used .…”

Use of 2n Gametes in Plant Breeding

“…As a general rule, it appears that approximately 0.1 to 2.0% of the gametes in a nonhybrid population are expected to be unreduced (Ramsey, 2007). However, the mean frequency of 2n gametes found in studies of hybrids (27.52%) was nearly a 50-fold greater than in nonhybrids (0.56%) (Ramsey and Schemske, 1998).…”