Summary Arnold, M. L., Kentner, E. K., Johnston, J. A., Cornman, S. & Bouck, A. C.: Natural hybridisation and fitness. – Taxon 50: 93–104. 2001. – ISSN 0040‐0262. There are several inferences that can be made from studies of the fitness of hybrid plants and animals. First, the fitness of hybrids varies. However, it is not possible to make a priori predictions concerning the relative fitness of a given hybrid genotype, or a series of genotypes. For example, plant hybrids are not more likely to have elevated fitness than animal hybrids. The variation in fitness for hybrids ranges from the highest fitness relative to parental genotypes to the lowest. The variation can be due to different environments or age classes. Furthermore, notwithstanding the paradigm that derived from the Neo‐Darwinian synthesis, some hybrid genotypes demonstrate elevated fitness relative to their parents. To test the evolutionary importance of hybridisation in a given species complex, it is critical that findings from greenhouse/population cage experiments be tested in the field. This latter statement is easier for a botanist to argue, but even with plants, the field experimentation needed for a rigorous test of fitness (i.e., reciprocal transplants) is at the best risky, and costly in time and resources. Yet, these are the types of studies needed to test predictions concerning the likeliest circumstances under which hybridisation will be promoted. The scientific literature over the past decade has seen a change in the tenor of papers describing natural hybridisation. Ten years ago, papers more often than not reported the use of natural hybridisation as a tool to understand divergent evolution. More recently, a majority of studies have discussed the evolutionary impact of natural hybridisation. We look forward to a continued increase in the frequency of studies that assume this process to have evolutionary importance per se.

Tandem repeat arrays often are found in interstitial (i.e., normally gene-rich) regions on chromosomes. In maize, genes on abnormal chromosome 10 induce the tandem repeats that make up knobs to move poleward on the meiotic spindle. This so-called neocentromere activity results in the preferential recovery, or meiotic drive, of the knobs in progeny. Here we show that two classes of repeats differ in their capacity to form neocentromeres and that their motility is controlled in trans by at least two repeat-specific activators. Microtubule dynamics appear to contribute little to the movement of neocentromeres (they are active in the presence of taxol), suggesting that the mechanism of motility involves microtubule-based motors. These data suggest that maize knob repeats and their binding proteins have coevolved to ensure their preferential recovery in progeny. Neocentromere-mediated drive provides a plausible mechanism for the evolution and maintenance of repeat arrays that occur in interstitial positions. INTRODUCTIONMany plants and animals have long arrays of tandem repeats in interstitial positions on chromosome arms (John and Miklos, 1979;Rodionov, 1999). Two such repeats in maize, one that is 180 bp and another that is 350 bp (TR-1), occupy condensed regions known as knobs (Peacock et al., 1981;Dennis and Peacock, 1984;Ananiev et al., 1998a). Knobs are found at 22 different positions in the karyotype and are strikingly polymorphic, making them excellent cytological markers (Longley, 1938;Kato, 1984). They also have the capacity to behave like centromeres, or "neocentromeres," in the presence of an unusual form of chromosome 10 ( Rhoades and Vilkomerson, 1942). In strains carrying normal chromosome 10 (N10), the knobs are quiescent, whereas in strains carrying abnormal chromosome 10 (Ab10), knobs at all positions in the genome move rapidly poleward on the meiotic spindle, dragging their chromosome arms with them (Rhoades and Vilkomerson, 1942). The mechanism of neocentromere activity remains a mystery, although it is known that neocentromeres lack two major kinetochore proteins, CENPC and MAD2 Yu, 2000), and interact with microtubules in a lateral manner instead of in the end-on manner typical of maize centromeres (Yu et al., 1997).Neocentromere activity plays an integral role in an associated phenotype known as meiotic drive. Meiotic drive has been documented in a variety of organisms (Lyttle, 1993), in which it is usually associated with several linked loci that collectively confer a segregation advantage to the linkage group. Meiotic drive systems presumably have evolved to "beat Mendel's rules" and therefore maximize their representation in the population (Sandler and Novitski, 1957). In some organisms, meiotic drive is a result of unusual chromosome segregation in meiosis (Rhoades, 1952;Cazemajor et al., 2000), and in others, it is caused by events that follow meiosis (Raju, 1996;Merrill et al., 1999). In maize, meiotic neocentromere activity at the large knob on Ab10 is thought to preferentially pull Ab10...

Morphological characters, chloroplast DNA, and allozymes were used to analyze the distribution of individuals within a hybrid population of the ferns Polystichum munitum and P. imbricans in northwestern California. Microsites within the population were characterized according to soil moisture and light levels reaching the plants. In sites with low soil moisture and high light levels, all of the ferns were genetically and morphologically like P. imbricans. In contrast, ferns with the genetic and morphological identity of P. munitum predominated in moist shady sites. Intermediate sites supported very few P.munitum, a wide variety of hybrid recombinants, and a majority of ferns with P. imbricans characteristics. The pattern of variation within the population is noteworthy because of the close proximity of the habitat extremes and the long-range dispersal of fern spores. We conclude that natural selection along environmental gradients must be a major factor in determining the ecological and genetic associations within the hybrid zone. The results of this study are evaluated in the context of the fern life cycle and compared to the assumptions of models explaining the establishment and maintenance of hybrid zones, which vary in the role attributed to environmentally mediated natural selection.

The Louisiana iris species Iris brevicaulis and I. fulva are morphologically and karyotypically distinct yet frequently hybridize in nature. A group of high-copy-number TY3/gypsy-like retrotransposons was characterized from these species and used to develop molecular markers that take advantage of the abundance and distribution of these elements in the large iris genome. The copy number of these IRRE elements (for iris retroelement), is ∼1 × 105, accounting for ∼6–10% of the ∼10,000-Mb haploid Louisiana iris genome. IRRE elements are transcriptionally active in I. brevicaulis and I. fulva and their F1 and backcross hybrids. The LTRs of the elements are more variable than the coding domains and can be used to define several distinct IRRE subfamilies. Transposon display or S-SAP markers specific to two of these subfamilies have been developed and are highly polymorphic among wild-collected individuals of each species. As IRRE elements are present in each of 11 iris species tested, the marker system has the potential to provide valuable comparative data on the dynamics of retrotransposition in large plant genomes.