Chromosome number evolution in skippers (Lepidoptera, Hesperiidae)

Abstract Lepidoptera (butterflies and moths), as many other groups of animals and plants, simultaneously represent preservation of ancestral karyotype in the majority of families with a high degree of chromosome number instability in numerous independently evolved phylogenetic lineages. However, the pattern and trends of karyotype evolution in some Lepidoptera families are poorly studied. Here I provide a survey of chromosome numbers in skippers (family Hesperiidae) based on intensive search and analysis of published data. I demonstrate that the majority of skippers preserve the haploid chromosome number n=31 that seems to be an ancestral number for the Hesperiidae and the order Lepidoptera at whole. However, in the tribe Baorini the derived number n=16 is the most typical state which can be used as a (syn)apomorphic character in further phylogenetic investigations. Several groups of skippers display extreme chromosome number variations on within-species (e.g. the representatives of the genus Carcharodus Hübner, [1819]) and between-species (e.g. the genus Agathymus Freeman, 1959) levels. Thus, these groups can be used as model systems for future analysis of the phenomenon of chromosome instability. Interspecific chromosomal differences are also shown to be useful for discovering and describing new cryptic species of Hesperiidae representing in such a way a powerful tool in biodiversity research. Generally, the skipper butterflies promise to be an exciting group that will significantly contribute to the growing knowledge of patterns and processes of chromosome evolution.


Introduction
The main karyotypic features of organisms, particularly the number of chromosomes, tend to be stable within species (White 1973, King 1993. New chromosomal rearrangements usually originate as heterozygotes and are often -although not always ) -associated with heterozygote disadvantage. The spread of such rearrangements to fixation within a large population has low probability (King 1993). Therefore, many organisms are characterized by chromosomal conservatism, a situation in which all closely related taxa demonstrate the same chromosome number.
In contrast to chromosomal conservatism, chromosomal instability characterizes situations where multiple closely related taxa (populations, subspecies and/or species) belonging to a single phylogenetic lineage differ drastically from each other by major chromosomal rearrangements, sometimes resulting in high variability in chromosome number.
Skippers (the family Hesperiidae) are studied to a lesser extent with the respect of karyotype evolution than the other butterfly families mentioned above (but see: Trew 1973, Saura et al. 2013). This family includes about 4000 species under 567 genera and is a globally distributed group found in all continents except Antarctica (Warren et al. 2008). The tribal level classification of skippers, based on combined analysis of molecular and morphological data, was recently elaborated by Warren and colleagues (Warren et al. 2008(Warren et al. , 2009.
Here I provide a first world-wide survey of chromosome numbers in skippers based on intensive search and analysis of published data.

Results
The results of literature search are presented in the Table below. It includes all the discovered chromosome counts except n=13 for Ochlodes venatus (Bremer et Grey, 1853), noted by Bigger (1960) as "Augiades venata". The name Ochlodes venatus was long used for the Ochlodes species of Europe, but it actually refers to its Far Eastern sister species, and the European taxon is now called O. sylvanus (Esper, 1777) (ICZN 2000). Both European and Far Eastern species have the same chromosome number n=29 (Federley 1938, Lorković 1941, Abe et al. 2006, not n=13 as indicated by Bigger (1960). Thus, the species name used by Bigger (1960) was probably misidentification.
The classification of skippers accepted in this paper follows Warren and colleagues (Warren et al. 2008(Warren et al. , 2009).

Modal chromosomal numbers
The table gives the chromosome numbers of 205 species of skippers, i.e. about 5% of the species of the world fauna. This number is not enough to infer any final statements about peculiarities of chromosome numbers distribution within the Hesperiidae. However, several tentative conclusions can be made. The haploid chromosome number n=31 was found in 50 studied species of skippers and, thus, it is a clear modal number for the family at whole. Interestingly, n=31 was found in representatives of all investigated subfamilies, except for Heteropterinae. However, in the last subfamily only one species was karyologically studied until now, and discovery of n=31 in Heteropterinae is not excluded in future. The next most common numbers are n=29 (43 species), n=30 (33 species) and n=28 (13 species).
Subfamilies Coeliadinae and Eudaminae have a sharp peak at n=31. In the subfamily Trapezitinae n=31 was also found (only one species studied).
Within the subfamily Pyrginae, the modal number n=31 is found in the tribe Erynnini. The tribe Pyrrhopygini is characterized by the most common n=28. The modal number in the tribe Tagiadini is n=30. The tribe Carcharodini has peaks at n=30 and n=31. In the tribe Pyrgini, n=29, n=30 and n=31 were found as the most common numbers.
In the family Heteropterinae n=29 was found (only one species studied). Within the subfamily Hesperiinae, the tribes Taractrocerini, Thymelicini, Calpodini, Moncini and Hesperiini are characterized by the most common n=29. Very variable chromosome numbers (from n=5 to n=50) were found in the tribe Aeromachini. It is difficult to infer the modal number for the last tribe. However, it should be noted that one species, Thoressa varia, has n=31 as the majority of other skippers. The tribe Baorini (subfamily Hesperiinae) has a clear peak at n=16, so it is exceptional with respect to the modal number of chromosomes.
The overall evidence indicates that chromosome numbers of Coeliadinae, Eudaminae, Trapezitinae, Pyrginae and Hesperiinae conform to the lepidopteran modal of n=31 (Robinson 1971). This number seems to be an ancestral one for the Hesperiidae as for the order Lepidoptera at whole (Suomalainen 1969, Lukhtanov 2000. This modal number (or its deviation to n=30, n=29 and 28) were preserved in the majority of skippers. However, in the tribe Baorini the number n=16 was evolved and, thus, represents a derived trait which can be used as a (syn)apomorphic character in further phylogenetic studies of the family Hesperiidae.

Between-and within-species variations in chromosome number
Several groups of skippers display extreme chromosome number variations at the within-species level (Table). The most extreme variations in number of chromosome elements were observed in first meiotic metaphase of Carcharodus boeticus, C. dravira and C. flocciferus (Table, de Lesse 1960). The nature of these variations remains unknown, and there are two plausible explanations for this phenomenon. First, this variation can be explained by presence of so-called B-chromosomes (=additional chromosomes, =supernumerary chromosomes) (de Lesse 1960). B-chromosomes consist mainly of repetitive DNA and can sometimes accumulate through processes of mitotic or meiotic drive (Jones et al. 2008). B-chromosomes can be distinguished from normal A-chromosomes because they are usually smaller and can be seen as additional chromosomes present in only some of the individuals in a population (Camacho et al. 2000, Jones et al. 2008). Second, this kind of variation can be caused by violations in meiotic chromosome pairing resulting in appearance of univalents (instead of bivalents) in meiotic prophase (Lorković 1990). This type of variation was studied in detail by Maeki and Ae (1979) in butterfly genus Papilio and is expected if regular or irregular interspecific mating occurs in nature. Anyway, the nature of intraspecific variations observed in Carcharodus is different from that discovered in the Wood White butterfly Leptidea sinapis (Linnaeus, 1758) (Pieridae). In the last species the compared range of intraspecific variation in chromosome number (from n=28 to n=53) was caused by multiple chromosome fusions/fissions accumulated within the species .
Between-species variation exists in numerous genera of skippers (Table 1) and is especially expressed in the Nearctic genus Agathymus Freeman, 1959, in which the range of haploid numbers was discovered from n =5 in A. aryxna to n=38 in A. alliae (Freeman 1969). This range is comparable of even exceeds the range found in chromosomally diverse genera from other butterfly families (Lorković 1990, Lukhtanov et al. 2005, Talavera et al. 2013. Thus, the genera of Hesperiidae can be used as model systems for future analysis of the phenomenon of chromosome instability.

Detecting cryptic species using analysis of chromosomal differences
Recent years karyological data have been widely used in studies of butterfly taxonomy and in biodiversity research as main or additional chracters for detecting cryptic species (e.g. Dinca et al. 2011) and for synonymizing biological entities that were incorrectly described as distinct species (e.g. Vila et al. 2010). The family Hesperiidae is not excluded in this respect. In the genus Gegenes Hübner, [1819], two cryptic species G. pumilio (n=24) and G. gambica (n=41) were discovered through extensive chromosome analysis of different populations (de Lesse 1960, 1967b, Larsen 1982, Saitoh 1984).
In the genus Pyrgus Hübner, [1819], our unpublished chromosome data (see Table) were used to recognize and then to describe two morphologically similar species, P. bolkariensis and P. aladaghensis (De Prins and van der Poorten 1995).
Thus, interspecific chromosomal differences are useful for discovering and describing new cryptic species of Hesperiidae representing in such a way a powerful tool in biodiversity research.