1. Introduction
The oriental river prawn
Macrobrachium nipponense is widely distributed throughout China [
1]. It is an important economical freshwater species and is cultivated due to its strong adaptability and disease resistance and high economic value [
2,
3].
M. nipponense has the characteristic of rapid sexual maturation. The breeding season of
M. nipponense is from April to October every year (with water temperature at 22 to 30 °C). The overwintering adult female prawns (BW ± SD: 0.78 ± 0.23 g, BL ± SD: 28.58 ± 3.35 cm) enter into the breeding season at the end of April, and its sexual maturation and embryo development will be accelerate by the rise in the water temperature. Every year, the newborn prawns from July in the pond can reproduce a large number of offspring in autumn (August to September, water temperature at 28 to 30 °C, BW ± SD: 0.50 ± 0.15 g, BL ± SD: 25.90 ± 2.12 cm) (commonly known as “autumn reproduction”). During the “autumn reproduction”, hatching to sexual maturation can take as little as 45 days, and rapidly maturing female prawns, with a body length as short as 3 cm, are able to lay eggs. This can result in multiple generations in a single pond, increasing food coefficient and hypoxia risk and reducing the ability of
M. nipponense to tolerate adverse conditions [
4]. “Autumn reproduction” can also aggravate cannibalistic behaviors. Such effects reduce the overall survival rate and the proportion of larger sized prawns. Therefore, research has focused on developing new strains of
M. nipponense characterized by fast growth but late maturation to avoid these negative impacts on
M. nipponense aquaculture.
Gonad development is the basis for the reproductive behavior of fishery resources, and it is closely related to the body size and age of individuals. In fish, most studies showed that first sexual maturation is related to body size (body length). Primary maturation begins when the size of the fish reaches or exceeds the minimum body length required for sexual maturity, and then the proportion of mature individual increases with the increase in body length. There is some research which indicated that first sexual maturity of fish, such as
Maccullochella peeli peeli and
Larimichthys crocea, were closely related to age [
5,
6,
7]. In
L. crocea, it has been proven to have a negative correlation with body weight, and selective breeding for the specific gonadosomatic index (GSI) is beneficial in improving the dressing percentage [
6,
7]. In crustaceans, there are a few reports about the relationship between first sexual maturity and body size and age, confirming that body size is more vital than age in
Penaeus schmitti,
Oratosquilla oratoria and
Trachysalambria curvirostris [
8,
9,
10]. In
T. curvirostris, the growth of individuals was slowed but did not stop completely during sexual maturity [
10]. In
M. nipponense, GSI had a significant negative effect on the size of female prawns [
11]. Therefore, elucidating the correlation between gonadal development and growth of fishery resources could provide a scientific basis for resource assessment and breeding management.
Crustacean gonad development occurs in two stages. The first relates to the early stage of first sexual maturity, which synchronizes with growth and involves complex changes in behavior, external morphology and gonadal morphology; by contrast, the second relates to periodic sexual maturity following adult maturation [
12]. Various factors, such as temperature, dietary and salinity, involved in the growth of
M. nipponense have been reported [
13,
14,
15,
16,
17]. In addition, research has focused on the genetic mechanisms of gonad development in
M. nipponense within numerous genes, such as those encoding vitellogenin (Vg), cathepsin (Mn-CTS L1) and legumain-like protease (Lel), being found to be closely related to rapid sexual maturation in this species [
4,
18,
19]. However, less research has focused on the relationship between growth and reproduction in
M. nipponense. Preliminary association analyses between gonad development and growth traits in randomly sampled
M. nipponense populations identified several SNPs based on Vg and associated with ovarian development and growth [
19]. Although these results provide a preliminary understanding of the relationship between the gonadal development and growth of
M. nipponense, how the variation between reproduction and growth is regulated was not fully elucidated.
In this study, we performed a correlation analysis between growth and gonad development during first sexual maturation of M. nipponense, and genetic effects studies of Cathepsin L gene on growth and gonad development were also carried out to find the SNPs for march-assisted selection. The results of this study will provide additional scientific background information for basic biological research of M. nipponense. These results could also accelerate selection of novel varieties of M. nipponense by providing effective tools for genetic improvement.
3. Discussion
Sexual maturation is an important transition from juvenile developmental growth to adult reproductive growth [
20]. The gonadosomatic index (GSI) is an economically important trait in many aquatic animals, which is related to the size of fishery resources and their sexual maturity. Growth trait parameters at sexual maturity are one of the key parameters for fishery resource assessment and risk assessment of management strategies [
21]. They are widely used in the analysis of growth pattern changes, estimation of maximum BL, and determination of minimum opening specifications of fishery resources, which provide an important reference for sustainable utilization of fishery resources [
22,
23,
24,
25]. Research has suggested that growth traits are affected by sexual maturity (including first sexual maturity and adult periodic sexual maturity), resulting in increasing pressure on resource utilization [
26]. A series of linear models and estimation methods have been established and are widely applied in fishery research [
26,
27,
28].
However, the correlation of gonadal development and growth was not same in different species. In crustacean
Oratosquilla oratoria, the gonad of large-size individuals developed significantly earlier than that of small-size individuals [
7]. In contrast, in fish, such as the large yellow croaker, body weight has been reported to show a very strong negative correlation (−0.96) with GSI [
9,
10]. Hence, selective breeding for specific GSI is beneficial in improving dressing percentage. In previous work, we used a randomly sampled juvenile
M. nipponense population to investigate the association between ovarian development and growth [
28]. The results showed that female GSI was negatively correlated with growth traits, and there were no significant correlations between male GSI and growth. These results were in accordance with the miniaturization observation in females with fast gonad development during autumn reproduction. However, the development of this randomly selected
M. nipponense population was non-synchronized, especially in the multigenerational population inhabiting the same aquaculture pond, making it difficult to fully assess the relationship between reproduction and growth. In the current study, full-sib families were used to investigate the association between growth and gonad development during first sexual maturation of
M. nipponense. The results indicate no significant correlations between ovarian development and growth traits, whereas male growth traits are positively affected by sexual maturity during growth in the first sexual maturation stage of
M. nipponense. The hepatopancreas, which is an important source of energy for gonadal development, showed no correlation with growth traits during first sexual maturation of
M. nipponense. These results also proved that the individuals with fast gonad development do not have a tendency to miniaturize. All the reports indicated that the effects of gonadal development on growth are the opposite in vertebrates and invertebrates, and its mechanism needs further research.
Using SNPs in association studies is a common strategy to screen the major gene and quantitative trait loci that regulate polygenic traits [
29,
30]. Significant associations between gene polymorphisms and a specific allele provide strong evidence that the genes are involved in regulating the target traits [
31]. Such an approach has been successfully applied in many aquacultural species of both fish and crustaceans [
32,
33,
34,
35,
36,
37]. Cathepsin Ls, which belongs to the lysosomal papain C1 family, is a cysteine protease with important roles in pathological and physiological processes, such as protein hydrolysis, antigen presentation, proteolysis and tumor metastasis [
38,
39]. Cathepsin L is the main enzyme involved in Vg degradation in insects [
40,
41] and has also been reported to be involved in Vg processing in zebrafish [
42]. In our previous study, the gene encoding cathepsin L1 in
M. nipponense (Mn-CTS L1) was characterized, and the RNAi results showed that it can promote ovarian maturation [
17]. In the current study, we investigated the association between specific alleles of Mn-CTS L1 polymorphisms and economically important traits of
M. nipponense.
The current results showed that four SNPs were significantly associated with all female growth traits, whereas two SNPs were significantly associated with male growth traits. In both the male and female populations, A+118T and A+1379C were strongly associated with all growth traits in M. nipponense. All three genotypes in A+118T showed much larger growth trait values compared with other loci in both male and female populations. Thus, A+118T might be a candidate SNP positively associated with larger growth traits. Further analysis showed that the number of SNPs significantly associated with gonadal development differed in males and females. In both male and female populations, A+1379C was strongly associated with all gonadal development traits. A+1379C showed multiple associated characteristics following a combined comparison of SNPs associated with growth and gonadal development traits. The CC genotype in A+1379C was positively associated with eight growth traits but negatively associated with GSI (p < 0.05). Thus, A+1379C could be applied as a potential molecular marker for gene-assisted selection to improve both reproduction speed and growth traits in M. nipponense.
Growth and reproductive performance are two of the most important biological indicators in crustaceans but are often studied in isolation. In this study, we focused preliminarily on the intrinsic genetic association between growth and reproduction by using full-sib families during the first sexual maturation stage of M. nipponense. We raised sib families in different cages in the same pond to reduce the impact of environmental and population differences on the results. However, this is only a preliminary study under ideal conditions. In practical aquaculture applications, environmental factors such as temperature, water quality and diet, which could significantly influence growth and reproductive traits, and expanding the genetic diversity and number of families studied will provide a more comprehensive understanding of the genetic factors at play. The complex polygenic nature of these traits can involve numerous genes and environmental interactions. These identified SNPs and their associations with desired traits are likely to be only a small part of a complex genetic network and should be validated in different populations and under varied aquaculture conditions to ensure their applicability in broader genetic improvement programs.
The studies about the impact of different temperatures, water quality and diets on growth and reproductive traits will be further strengthened, and the underlying genetic mechanisms will be investigated. Diverse genes and large-scale family samples will be gradually introduced to give a more comprehensive understanding of the role of genetic factors in growth and reproductive performance. All the findings will be validated in different populations and under varied aquaculture conditions to make them more generalizable and applicable. Moreover, detailed morphological, physiological or behavioral aspects should be included to provide a more complete understanding of the genetic variation involved in
M. nipponense biology and ecology. Future studies could benefit from using AI technologies to integrate genetic data with environmental and morphological analyses, offering a holistic view of the factors influencing
M. nipponense growth and reproduction [
43,
44]. Finally, the identification of potential SNPs for marker-assisted selection is only the first step in aquatic genetic improvement; more considerations, such as cost, ethical implications of genetic selection and the balance between improving specific traits versus the overall health and resilience of populations, should be considered before further practical applications.