Mezzerin_06.indd UDC 575.17:595.14 COMPARATIVE ANALYSIS OF FECUNDITY IN RELATED AMPHIMICTIC APORRECTODEA CALIGINOSA AND APOMICTIC A. TRAPEZOIDES EARTHWORMS, AND THE PROBLEM OF REPRODUCTIVE ADVANTAGES OF PARTHENOGENETIC ANIMALS S. V. Mezhzherin1, Yu. Yu. Chayka2, S. V. Kokodiy1, A. A.Tsyba1 1Schmalhausen Institute of Zoology NAS of Ukraine, vul. B. Khmelnytskogo, 15, Kyiv, 1030 Ukraine E-mail:s.mezhzherin@gmail.com 2Zhytomyr State Ivan Franko University, Velyka Berdychivska st., 40, Zhytomyr, 10008 Ukraine S. V. Mezhzherin (https://orcid.org/0000-0003-2905-5235) Yu. Yu. Chayka (https://orcid.org/0000-0002-3965-6088) S. V. Kokodiy (https://orcid.org/0000-0002-0651-6935) A. A.Tsyba (https://orcid.org/0000-0001-5838-0948) Comparative Analysis of Fecundity in Related Amphimictic Aporrectodea сaliginosa and Apomictic A. trapezoides Earthworms, and the Problem of Reproductive Advantages of Parthenogenetic Animals. Mezhzherin, S. V., Chayka, Yu. Yu., Kokodiy, S. V., Tsyba, A. A. — Th e comparative analysis of fecundity and fertility was studied experimentally for the amphimictic Aporrectodea caliginosa diploid and the close parthenogenetic A. trapezoides triploid earthworms during two seasons. Th e i n dividual fecundity of cocoons at is signifi c antly higher in the parthenogenetic species than in amphimictic one. Fertility is in contrast lower in the parthenogenetic species, which results in leveled parameters of the reproductive potential. A generalization and analysis of the available data on the comparative fecundity of representatives of diff e r ent animal groups shows that the automatic increase in fecundity in same- sex organisms due to the exclusion of males cannot be considered a universal rule providing biological progress and the ecological advantage of parthenogenetic organisms. Th i s explanation is not suitable for hermaphroditic organisms. In addition, in some cases, parthenogenetic reproduction is accompanied by reduced fertility and even reduced fecundity. K e y w o r d s : parthenogenesis, fecundity, earthworms, biological press Zoodiversity, 54(6): 479–486, 2020 DOI 10.15407/zoo2020.06.479 Ecology 480 S. V. Mezhzherin, Yu. Yu. Chayka, S. V. Kokodiy, A. A.Tsyba Introduction Th e relationship of apomictic and amphimictic reproduction in biological evolution remainsy one of the unresolved issues of modern biology. Th e logic of the evolutionary transition from asexual to sexual reproduction is not completely clear, and the most important and obscure points of interest are the meaning of irreversible rejection of hermaphroditism and the transition to obligate bisexual reproduction. In the evolutionarily most advanced groups, in particular Cephalopoda, Arthropoda and Chordata, the hermaphroditism is present only as sporadic pathologies, and the rare situations of asexual reproduction are either random event (monozygotic twins), or a useful device (polyembryony). Parthenogenesis is the sexual reproduction by unfertilized eggs (Cuellar, 1977; Suomalainen et al., 1987), which holds a special place in the reproductive system of higher animals. Th is mode of reproduction should be considered secondary in relation to amphimixis, and therefore a progressive method of sexual reproduction. Parthenogenesis is believed to provide short-term benefi ts. Several types of parthenogenesis are known. One of the most common forms of parthenogenesis is associated with the formation of organisms with an allopolyploid structure of the genome (Suomalainen et al., 1987). Th ey originate from interspecifi c crosses, which result in the formation of polyploid same-sex hybrids, in which the gametogenesis occurs without the reduction divisions. Th e fi rst-generation hybrids, usually females, produce diploid gametes, which are fertilized by the spermatozoa of males of parental species. Th at leads to the formation of triploids that can multiply by parthenogenesis (Suomalainen et al., 1987). Th e tetraploids appear in the following hybridizations. In some groups, like earthworms (Mezhzherin et al., 2017), even octoploids can be found. Th e alloploid parthenogenetic species and biotypes are found in almost all large taxa of freshwater and terrestrial animals of the Holarctic. Th e exception is birds and mammals. In the case of mixed populations, parthenogenetic specimens are usually not inferior in abundance to parental species, and in a situation of isolation from parental species, their populations are no less numerous. Th ey are capable of large-scale expansion, subsequently occupying the territories and landscapes that are not suitable for amphimictic species (Cuellar, 1977; Suomalainen et al., 1987). What is the reason for the success of same-sex parthenogenetic specimens? Many researchers have now agreed upon their high reproductive potential at the population level as the key factor (Williams, 1975; Cuellar, 1977; Maynard Smith, 1978; Suomalainen et al., 1987; Dawson, 1995; Gibson et al., 2017). In the initially hermaphroditic organisms, the absence of spermatogenesis probably allows to focus the life potential of individuals on egg production. In the initially bisexual organisms, the absence of males allows at the population level to save up resources intended for their production. Th us, ceteris paribus, the parthenogenetic populations should theoretically be more prolifi c than amphimictic. Nevertheless, this seemingly logical assumption remains only a hypothesis which is not supported by suffi cient evidence. Especially when it comes to the hermaphroditic groups of animals in which parthenogenesis is especially common. Th e idea that the energy saved up in the event of rejection of spermatogenesis in these primitive animals can be used to increase the production of oocytes and to enhance the reproductive potential is purely theoretical and does not take into account several circumstances. First of all, this concerns the quality of ameiotic eggs and the viability of parthenogenetic off spring. Specifi c experiments have shown that the level of egg production in planarians of the family Dugesiidae (Weinzierl et al., 1999) and oligochaetes of the family Tubifi cidae (Poddubnaya, 1984) is initially equal or higher compared to the related amphimictic forms. However, the survival of hermaphroditic off spring is much lower. A steady decrease in individual fecundity is also characteristic of some parthenogenetic insects (Roth, 1974; White, Contreras, 1979) and gynogenetic fi sh (Kokodiy, 2016; Mezhzherin et al., 2017; Przybyl et al., 2019). At the same time, there is a number of data of the opposite nature, confi rming the real possibility of increasing the reproductive potential in populations of parthenogenetic organisms primarily due to the same-sex population structure (Weeks, 2005; Crummett, Wayne, 2009; Tada, 2013; Schall, 1981; Schlupp et al., 2010, etc.). Th e inconsistency of the data indicates a limited explanation for the evolutionary success of parthenogenetic organisms based only on their reproductive advantages. A better explanation requires both additional research and theoretical analysis and synthesis of available data on comparative fecundity. Particular attention should be paid to earthworms, in which parthenogenesis is very common, and data on comparative fecundity are not available. Th e aims of our work were to conduct a comparative analysis of the fecundity of the close amphimictic and apomictic earthworm species of the genus Aporrectodea in laboratory conditions, and to generalize and subsequently analyze materials on the comparative fecundity of similar groups of animals. Material and methods A comparison of the individual fecundity of parthenogenetic and amphimictic earthworms under controlled conditions was carried out on two closely related species of Aporrectodea Orley, 1885. A. caliginosa (Savigny, 1826) has a diploid genome and an amphimictic reproduction system. It is the most widespread species of the open landscapes of the forest and forest-and-steppe zones of Europe. A. trapezoides   (Dugés, 1828) is parthenogenetic, has a triploid genome and a more southerly distribution. Most likely, this allopolyploid species occurred as a result of hybridization of A. caliginosa with an unknown species of that genus (Mezhezherin et al., 2018). In the forest-steppe zone, the ranges and habitats of the species overlap (Perel, 1979). 481Th e Comparative Analysis of Fecundity in Related Amphimictic Aporrectodea сaliginosa and Apomictic... Th e study was conducted over two seasons. In 2018, 30 mature earthworms were used as the material for the study. Ten of them were A. trapezoides and 20 were identifi ed as A. caliginosa. Th e earthworms were kept in pairs. Th e sample of A. trapezoides was taken in the fi rst half of April from the Belichi housing district in Kyiv (50.45955, 30.35166), and the sample of A. caliginosa was collected in the vicinity of Baranivka village (50.29639, 27.67111) of Zhytomyr Region. In 2019, fecundity was analyzed in 27 A. trapezoides and 42 A. caliginosa specimens. Th e parthenogenetic earthworms were kept one specimen per container, and the amphimictic were kept in pairs. Earthworms were collected in the fi rst half of April from three places in Zhytomyr Region: near Stanishovka village (50.21982209, 28.72116799), in the district of Maryanovka, Zhytomyr city (50.28711826, 28.70536566) and from Radomyshl town (50.50361, 29.24611). An original technique was developed to culture earthworms of the genus Aporrectodea (Chayka, Vlasenko, 2019). Th e technique allows a satisfyingly correct evaluation of the number of cocoons and juvenile forms kept in small containers. Optimum soil moisture and a completely nutritive diet were previously selected to do that. Th e earthworms were kept without light at room temperature in well-ventilated plastic containers with a volume of 0.33 l. Th e soil used for cultivation was taken directly from the collection site and pre-sieved through a zoological sieve. Th e experiments started in mid-April and ended in late summer to early autumn, when the breeding intensity faded away, and the parental specimens were not viable in a number of containers. Th e number of cocoons and juvenile individuals was counted by sieving the substrate every 5–10 days depending on the intensity of reproduction. Th e removal of young earthworms was not carried out. Fecundity parameters were calculated per one mature adult. Two parameters were used, which refl ect the mean number of cocoons or larvae produced by one specimens during the reproductive season. Th e mean group score is based on the mean numbers of cocoons and juveniles counted on a particular day. An individual assessment is based on the summarized counts of cocoons and juveniles in each container, carried out over the entire period of the study. In containers containing two worms, individual fecundity was assessed as the mean value obtained for two parental specimens. Situations when an individual A. caliginosa remained in the container were not taken into account. Results Th e process of laying cocoons in individual specimens began in 2018 on 20th day of experiment, and in 2019 on the 8th day of experiment, simultaneously in two species. Th e larvae appeared later in 2018 aft er 20 days, and in 2019 aft er 40 days from the start of the experiment, also at the same time in containers with A. caliginosa and A. trapezoides. Reproduction peaked in June. In July, the number of produced cocoons decreased; in August, despite stable humidity and high temperature, reproduction almost stopped (fi g. 1, 2). Counts of the number of cocoons showed that the individual fecundity of A. trapezoides was signifi cantly higher than that of A. caliginosa. Th e results obtained in diff erent seasons and with diff erent calculation methods showed that the mean number of cocoons per specimen in the parthenogenetic species is signifi cantly higher than in the amphimictic species (table 1). Notably, the cocoons of diff erent species did not diff er in size, shape and color. Dissection showed that A. caliginosa cocoons contained one embryo, whereas in A. trapezoides their number ranged from one to two. Th e greatest degree of discrepancy in the fecundity levels of earthworm species is observed in a period of maximum breeding intensity. In most counts, conducted during this period, diff erences in the number of cocoons per individual were statistically signifi cant (table 2). At the beginning and at the end of the season, when the number of laid cocoons is minimal, diff erences in fecundity are almost absent (fi g. 1, 2). T a b l e 1 . Mean number of cocoons per specimen in amphimictic A. caliginosa and parthenogenetic A. trapezoides earthworms in culture conditions Year Statisticalparameters Mean number of cocoons per one parental specimen Mean group assessment Individual assessment A. caliginosa A. trapezoides A. caliginosa A. trapezoides 2018 M ± SE 0.27 ± 0.04* 1.16 ± 0.16* 0.25 ± 0.04* 1.18 ± 0.18* n 7 7 79 34 2019 M ± SE 2.51 ± 0.44 3.05 ± 0.6 2.47 ± 0.12** 3.07± 0.14* n 15 15 324 415 N o t e . M ± SE — mean value and standard error, n — number of counts for group assessment and the number of individual assessments. *Diff erences are signifi cant at р < 0.001. 482 S. V. Mezhzherin, Yu. Yu. Chayka, S. V. Kokodiy, A. A.Tsyba 0 1 2 3 4 5 6 7 8 N o of c oc oo ns p er a du lt sp ec im en A. trapezoides A. caliginosa Fig. 2. Mean number of cocoons per one mature specimen in close parthenogenetic (A. trapezoides) and amphimictic (A. caliginosa) earthworm species during the reproduction season of 2019. T a b l e 2 . Comparative analysis of individual fecundity assessed as cocoon laying in two earthworm species during maximal productivity Date A. trapezoides A. caliginosaM ± SE M ± SE June 04, 2019 6.46 ± 0.41** 4.57 ± 0.45** June 11, 2019 6.96 ± 0.52* 5.10 ± 0.51* June 17, 2019 6.70 ± 0.50* 5.10 ± 0.51* June 26, 2019 5.70 ± 0.55 4.38 ± 0.46 N o t e . M ± SE — mean value and standard error. Diff erences are signifi cant at: *р < 0.05, **p < 0.01. 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 Apr-10 Apr-23 May-07 May-21 Jun-04 Jun-18 Jun-26 Jul-07 Jul-20 Jul-29 A. trapezoides A. caliginosa Fig. 1. Mean number of cocoons per one mature specimen in close parthenogenetic (A. trapezoides) and amphimictic (A. caliginosa) earthworm species during the reproduction season of 2018. Fertility levels depend not only on the earthworm species, but also on the population from which they are taken. Nevertheless, in all cases, specimens of the parthenogenetic species produce a greater number of off spring (table 3). Comparison of two species by the mean number of juvenile specimens, calculated per parent specimen, yielded signifi cant diff erences between amphimictic and apomictic worms T a b l e 3 . Mean number of cocoons per parental individual in two species of earthworms of diff erent populations in 2019 Samples A. caliginosa A. trapezoidesM ± SE n M ± SE n Stanishevka 2.58 ± 0.19* 96 3.11 ± 0.15* 384 Maryanovka 3.20 ± 0.19 160 3.46 ± 0.61 16 Radomyshl 1.53 ± 0.17 80 1.97 ± 0.33 32 N o t e. *Diff erences signifi cant at р < 0.05. 483Th e Comparative Analysis of Fecundity in Related Amphimictic Aporrectodea сaliginosa and Apomictic... in only one case out of four comparison options (table 4). Moreover, these diff erences were at the lowest level of signifi cance. Th e reason for the lack of diff erences at the level of juvenile specimens is most likely the relatively low survival rate of A. trapezoides in the early stages compared to A. caliginosa. Discussion of results Summarizing the study results, it can be unequivocally stated that the parameters of individual fecundity based on counts of the number of deposited cocoons are higher in the parthenogenetic triploid species A. trapezoides than in the amphimictic diploid A. caliginosa. At the same time, at the level of juvenile specimens, the reproductive potential is leveled. Th at is associated with the relatively low viability of A. trapezoides, which is quite possibly caused by artifi cial conditions. In any case, this means that meiosis is not always the most eff ective way to produce gametes in animals, and amphimixis is not the most eff ective form of reproduction. Th e mitotic gamete production and apomixis may well replace them. Th e fi ndings on the comparative fecundity of genetically close amphimictic and apomictic species of earthworms are consistent with the results of similar studies conducted on other groups of hermaphroditic invertebrates. For example, the study on planarians of the Schmidtea polychroa group (Weinzierl et al., 1999) and two species groups of oligochaetes of the Tubifi cidae family (Poddubnaya, 1984). In the fi rst case, the fecundity is initially higher, compensated by the low fertility of cocoons and off spring. In the second case, with the initially equal fecundity of amphimictic and parthenogenetic worms, the latter also have a sharp decrease in the survival of off spring during the life cycle. In primitive hermaphrodite animals, where parthenogenesis is very common, this means that, ceteris paribus, apomixis not only does not lead to a greater reproductive potential, but also limits it compared to similar amphimictic species. Nevertheless, the increase in reproductive potential in animal species with obligate parthenogenetic is undeniable. An increased production of oocytes at the population level can be noted for most primarily bisexual animals (table 5). Th at increase is achieved, fi rst of all, due to the absence of males. Th is fact is evident in the gastropod mollusks Campeloma limum (Crummett, Wayne, 2009) and Pomatopyrgus antipodarum (Schreiber et al., 1998), among insects in Clitarchus hooker sticks (Morgan-Richards, Trewick, 2010) and in Scepticus insularis weevils (Tada, Katakura, 2013). Among vertebrates, a similar situation occurs in cyprinids of the Poecilia formosa groups (Schlupp et al., 2010) and Poeciliopsіs monacha– lucida (Weeks, 2005), the salamander species complex Ambystoma tigrinum (Bogart et al., 1987) in lizards of the genera Cnemidophorus and Aspidoscelis of the family Teidae (Schall, 1981; Newton et al., 2016), in geckos Heteronotia binoei, Gekkonidae (Kearney, Shine, 2005), in Caucasian lizards of the genus Darevskia, Lacertidae (Darevsky, 1967) of the order Squamata. In all these cases, the fecundity of parthenogenetic allopolyploids does not exceed that of the parent or close amphimictic species. Due to the absence of males, the allopolyploids are most likely to achieve a greater reproductive potential. T a b l e 4 . Mean number of juvenile specimens per one parent in amphimictic A. caliginosa and parthenogenetic A. trapezoides earthworms in culture conditions Year Statisticalparameters Mean number of juvenile specimens per parent specimen Mean group assessment Individual assessment A. caliginosa A. trapezoides A. caliginosa A. trapezoides 2018 M ± SE 0.760 ± 0.045 1.271 ± 0.210 0.77 ± 0.084* 1.221 ± 0.157* n 7 7 63 34 2019 M ± SE 1.092 ± 0.171 1.258 ± 0.210 1.071 ± 0.02 1.249 ± 0.099 n 10 10 198 415 N o t e. Statistical parameters are same as in table 1. *Iiff erences s ignifi cant at р < 0.05. 484 S. V. Mezhzherin, Yu. Yu. Chayka, S. V. Kokodiy, A. A.Tsyba Th ere are proven exceptions to that rule. Th us, in parthenogenetic cockroaches of the Pycnoscelus indicus–surinamensis group (Roth, 1974) and the grasshopper Warramba virgo (White, Contreras, 1979), parthenogenetic females are characterized by reduced fecundity, which, however, does not aff ect their ecological success. T a b l e 5 . Comparative analysis of fecundity, fertility and reproductive potential of populations of parthenogenetic species (forms) with related amphimictic species Species / Group of species Fecundity and Fertility Repro- ductive potential Literary issues Turbellaria, Dugesiidae Schmidtea polychroa Same-sex forms have higher fecundity of cocoons and low fertility < Weinzierl et al., 1999 Oligochaeta, Lumbricidae Aporrectodea caligino- sa–trapezoides Parthenogenetic species have a greater number of cocoons with lesser survival of juveniles = Th is article Oligochaeta, Naididae Tubifex tubifex With an equal number of cocoons, a sharp decrease in survival in parthenogenetic forms during the life cycle < Poddubnaya, 1984 Limnodrilus hoff meisteri With an equal number of cocoons, a sharp decrease in survival in parthenogenetic forms during the life cycle < Poddubnaya, 1984 Gastropoda, Viviparidae Campeloma limum Parthenogenetic forms are more prolifi c > Crummett, Wayne, 2009 Gastropoda, Hydrobiidae Pomatopyrgus antipo- darum An equal fecundity > Schreiber et al., 1998 Insecta, Blattoptera, Blaberidae Pycnoscelus indicus– surinamensis Fecundity in parthenogenetic species is below than amphimictic one < Roth, 1974 Insecta, Orthoptera, Morabitidae Warramba virgo With an equal number of clutches, partheno genetic forms have a half number of eggs < White, Contreras, 1979 Insecta, Coleoptera, Curculionidae Scepticus insularis Parthenogenetic forms are more fertile > Tada, Katakura, 2013 Insecta, Phasmatodea, Phasmatidae Clitarchus hooker An equal fecundity > Morgan-Richards, Trewick, 2010 Actinopterygii, Cyprinodontiformes, Poecilidae Poecilia formosa–latipi- na–mexicana An equal fecundity > Schlupp et al., 2010 Poeciliopsis monac- ha —lucida An equal fecundity > Weeks, 2005 Actinopterygii, Cypriniformes, Cyprinidae Carassius auratus–gi- belio Fecundity in parthenogenetic species is lower by 30 % = Kokodiy, 2016; Przybyl et al., 2019 Actinopterygii, Cypriniformes, Cobitidae Cobitis elongatoides– taenia–tanaitica Fecundity in parthenogenetic species is lower by 30–40 % , and tetraploids by 70–80 % = Mezhzherin et al., 2017 Amphibia, Caudata, Ambistomatidae Ambistoma tigrinum complex An equal fecundity > Reptilia, Squamata, Teiidae Cnemidophorus An equal fecundity > Schall, 1981 Aspidoscelis An equal fecundity > Newton et al., 2016 Reptilia, Squamata, Gekkonidae Heteronotia binoei An equal fecundity in nature > Kearney, Shine, 2005 Reptilia, Squamata, Lacertidae Darevskia An equal fecundity Darevskiy, 1965 N o t e . Th e reproductive potential of populations of parthenogetic species: greater (>), less () and equal (=), compared with related amphimictic species. 485Th e Comparative Analysis of Fecundity in Related Amphimictic Aporrectodea сaliginosa and Apomictic... A special situation occurs in gynogenetic polyploid cyprinids of the families Сobitidae and Сyprinidae. In triploid hybrid fi sh of the Cobitis elongatoides– taenia– tanaitica group, the production of eggs is less by one third than in individuals of the diploid parent species (Mezhzherin et al., 2017), and in tetraploid fi sh, the egg production is only 20 % of the diploid level. Th e reason for the decrease in fertility is a sharp increase in the size of eggs in polyploids. A similar situation was also observed in triploid fi sh of the Carassius auratus– gibelio group, whose gynegenetic females, as shown by studies in Ukraine (Kokodiy, 2016) and Poland (Przybyl et al., 2019), also increased egg size and, accordingly, reduced individual fecundity. Th is means that, taking into account the 30–40 % ratio of males in populations, the reproductive potential of the gynogenetic representatives of these species groups is close to the level of amphimictic. It should also be noted that high fecundity rates of parthenogenetic forms may not refl ect the real reproductive potential of the population, since the survival of clonal off spring is oft en lower than that of amphimictic. Special studies on this problem have not been conducted for most objects, although available data, for example, on Poecilia formosa show (Hubbs, Schlupp, 2008) that survival in the early stages of the development of a hybrid gynogenetic form is not diff erent from that of a parent species. Th us, the hypothesis that the transition from bisexual to same-sex reproduction should automatically lead to an increase in the reproductive potential of populations of parthenogenetic species is not supported by specifi c studies on comparative fecundity. Th is means that the hypothesized rule is not universal and can not be extended to diff erent systematic groups. First, it is in principle not suitable for hermaphroditic organisms, among which parthenogenesis is widespread. Second, in some cases there is a decrease in the fecundity of same-sex species or forms compared with similar bisexual species. Th ird, in some systematic groups, there is a low fertility of parthenogenetic off spring. Th is situation is more likely an exception, but it gives every reason to believe that the postulated increase in reproductive potential in unisexual species compared with bisexual species is not a universal rule, but as a particular mechanism of evolutionary success. 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Cytogenetics of the parthenogenetic grasshopper Warramba (formerly Moraba) virgo and its bisexual relatives. V. Intercation of W. virgo and a bisexual species in geographic contact. Evolution., 33 (1), 85–94. Received 30 June 2020 Accepted 15 December 2020