is the agricultural land of the Midwest. Given the intense farming practices used and the nearly constant changes Weedy Amaranthus species pose a serious threat to agriculture.in Best Management Practices, agrichemicals used, etc., One of the areas that causes the greatest concern is development of herbicide resistance and subsequent transfer of herbicide resistance the landscape is in a continual state of flux. If interspegenes among the species. To determine the potential impact of inter-cific hybridization does occur in Amaranthus, introgresspecific hybridization, one must first be able to detect such hybrids.sive hybridization could be very important in the evolu-To determine if genome size could be useful in the detection of intertion and adaptation of these weedy species. specific hybrids among the weedy Amaranthus species, the genome Murray (1940) reported that hybrids could be made sizes of the weedy Amaranthus species sympatric in Illinois were anaamong various monoecious and dioecious pigweed spelyzed. In a series of experiments, the genome sizes of these species cies. Wetzel et al. (1999) and Franssen et al. (2001) rewere determined by flow cytometric analysis. A significant variation ported the production of interspecific hybrids between was observed with respect to nuclear DNA content in the eight species two dioecious pigweed species. Tranel et al. (2002) reexamined. The genome sizes ranged from ≈0.95 pg in A. palmeri to ported interspecific hybridization between a monoe-≈1.4 pg in A. tuberculatus. Overlap of genome sizes among the species does exist; however, due to the reproductive biology of the species, cious and a dioecious pigweed species. In all the above this overlap does not preclude the detection of interspecific hybrids.reports, the hybrids were made under controlled, iso-Any dioecious weedy Amaranthus plant in Illinois that has a genome lated conditions. Trucco et al. (2005) demonstrated insize between 1.3 pg and 1.1 pg is probably a hybrid. Thus, the potential terspecific hybrids between A. hybridus and A. tubercufor genome size determination to reveal interspecific hybrids among latus under field conditions. Thus, it is possible that weedy Amaranthus species has been demonstrated. hybrids between pigweeds could occur naturally in agronomic fields. size to be 5.35 pg per 2C nucleus. Given the almost iden-MH 262 MH 262 Maricopa, AZ tical estimates of DNA, all the weed populations were con-MH 277 MH 277 Beckham, OK MH 254 Brazos, TX verted using 5.35 pg per 2C nucleus for W22 to maintain A. albus PT 66 PT 66 Champaign Co., IL consistency with other data published using W22 as an PT 106 PT 106 Morgan Co., IL internal standard. MH 40 MH 40 Dona Ana, NM MH 49 MH 49 Stoneville, MS The genome size of the pigweed species ranged from MH 55 MH 55 Centre Co., PA 0.96 to 1.21 pg per 2C nucleus in experiment 1 and 0.95 to A. blitoides MH 215 MH 215 Whitman Co., WA 1.23 pg per 2C nucleus in experiment 2 (Table 2). In MH 219 Franklin Co., MD MH 206 Harrow, Ontario both experiments, species were signif...

Summary Amaranthus tuberculatus represents one of the most dramatic cases of weed invasion documented in the midwestern USA. The species is infamous for evolving resistance to multiple herbicides, and predicting whether these resistances may be transferred to widespread weeds of the Amaranthus hybridus aggregate is a matter of epidemiological concern. Here, we explore the patterns of genetic exchange between Amaranthus tuberculatus and A. hybridus in an effort to understand whether allele introgression occurs throughout the genome and if fecundity penalties are associated with genetic exchange. We evaluated 192 homoploid BC1s at 197 amplified fragment length polymorphism (AFLP) loci, as well as two loci associated with herbicide resistance: ALS and PPO. We also assessed the fecundity of each genotype by evaluation of seed production or pollen development. It was discovered that genetic exchange between the species is unidirectional. Whereas A. hybridus alleles transfer with little or no penalty to A. tuberculatus, the reciprocal exchange is significantly distorted and potentially of limited evolutionary consequence. Our previous hypothesis suggesting unidirectional introgression at ALS owing to circumstantial linkage is now modified to account for the more generalized distortion of genetic exchange observed in this study.

The nuclear DNA content for a group of 40 Malus species and hybrids has been estimated using flow cytometry. Estimates of nuclear DNA content for this germplasm collection range from 1.45 pg for Malus fusca (diploid) to 2.57 pg for Malus ioensis (triploid). Among diploids, the nuclear (2C) DNA ranges from 1.45 pg for M. fusca to 1.68 pg for Malus transitoria. Among triploids, the nuclear (3C) DNA content ranges from 2.37 pg / 3C for Malus sikkimensis to 2.57 pg / 3C for M. ioensis. Given the complexity of the apple genome and its suggested allopolyploid origin, the results obtained in this study confirm earlier reports that polyploids can easily withstand the loss of a certain amount of DNA, and that there is a slight tendency towards diminished haploid nuclear DNA content with increased polyploidy.

Large proportions of nonhybrid progeny result from controlled crosses between Palmer amaranth and common waterhemp, both dioecious weeds. Agamospermy was proposed as an explanation for this phenomenon, and here we provide evidence in support of this hypothesis. We evaluated 60 nonhybrid offspring from two interspecific crosses, and all plants were females and had DNA content values similar to the female (Palmer amaranth) parent. Among nine hybrids resulting from these crosses, eight were nonviable (lethal or neuter), and only one hybrid allowed for continued gene movement. Cytogenetic evaluation of this hybrid revealed triploidy, further supporting the occurrence of unreduced gametes in these species. In light of this new evidence, we examine earlier data regarding Palmer amaranth by common waterhemp hybridization and suggest some prior conclusions may be premature.

Pumpkins, a subgroup of the domesticated Cucurbita species, have been reported to range in fruit type (related to size) from miniature (<100 g) to jumbo (>273 kg). In order to obtain a wide range of fruit types it is hypothesised that all potential factors affecting fruit type must be used. One factor that is often overlooked in plant studies is genome size. In various plant species, genome size variation has been associated with characteristics such as cell size, plant size and flowering time. Such characteristics are referred to as nucleotypic parameters. In order to determine if nucleotypic selection is occurring in pumpkin, 17 varieties were analysed for genome size variation in two separate experiments. The species selected encompass the total range of fruit types reported in pumpkin. Significant nuclear DNA content variation was observed in pumpkin. There was no significant correlation between genome size and fruit type. In fact, the miniature pumpkin types were found to have the same genome size as the jumbo pumpkin types. In addition, a positive correlation between genome size and stomata length (an estimate of cell size in plants) was observed. Both the miniature and jumbo types were observed to have the smallest genome size and the smallest cell size. Thus, nucleotypic selection does appear to occur in pumpkin and appears to be involved in determining fruit type, although it may not be the only factor involved.